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Personal signature is used to sign contracts. However it is impossible to prove the veracity of a signature, i.e. graphology is not scientific (see http://psycnet.apa.org/record/1998-07238-000)
Why are signatures widely used when one could use fingerprints to "sign" contracts?
The historical and cultural context described by @ohwilleke is really the major reason why signatures are widely used to endorse documents, and fingerprints are not.
However the question assumes that signatures cannot be verified with reliability. See https://www.crimemuseum.org/crime-library/forensic-investigation/handwriting-analysis/ and https://www.researchgate.net/profile/Marcus_Liwicki/publication/221389470_Forensic_Signature_Verification_Competition_4NSigComp2010_-_Detection_of_Simulated_and_Disguised_Signatures/links/0c960532ab0387baf0000000.pdf Forensic handwriting analysis is quite different from graphology. The latter attempts to determine personality characteristics from handwriting, and is indeed discredited.
According to this Wikipedia article:
Graphology (or graphoanalysis, but not graphanalysis) is the analysis of the physical characteristics and patterns of handwriting claiming to be able to identify the writer, indicating psychological state at the time of writing, or evaluating personality characteristics. It is generally considered a pseudoscience. The term is sometimes incorrectly used to refer to forensic document examination, due to the fact that aspects of the latter dealing with the examination of handwritten documents are occasionally referred to as graphanalysis.
Moreover, it is by no means the case that fingerprint impressions cannot be forged. As long ago as 1907, R. A.Freeman, in The Red Thumb Mark showed that it was possible for someone with access to an actual fingerprint impression of a person, to create a stamp which would replicate that impression exactly. But with current technology, anyone with a document signed with a fingerprint could scan the print, and the add the print as an image to any other document. Such a forged print would be, if anything, harder to detect than forged handwriting.
You would be amazed at how vanishingly few the number of cases are where a signature is disputed.
Signatures are easy, quick and don't require you having inky fingers all the time. They are so useful that to throw them out to deal with infinintesimally small fractions of disputes over their veracity (bearing in mind that 99.999999999% of contracts never have a dispute that gets to a court [or at all]) is ridiculous.
When it does happen, handwriting analysis is probably not going to be put into evidence anyway. Testimony like "I saw him sign it" is way more likely to be used.
Fingerprinting as a reliable and widely acknowledged means of identification only dates to about 1879 CE. Signatures have been in wide use for many, many centuries and were preceded by the use of official seals (a practice still in use in Japan). So, the use of signatures was deeply entrenched in the legal system long before fingerprinting existed, and fingerprinting never came to displace signatures because, as observed by @DaleM, signatures are rarely disputed.
The other aspect of a signature, which can be, but is not necessarily shared by a fingerprint, is that it has acquired a culture meaning as a definitive sign that the person signing something endorses it to be their intentional act. In contrast, the meaning of a document that is not signed is often ambiguous. It could be a draft that was never adopted by the purported author as a definitive expression of their intent, for example.
The most widely adopted substitute for a signature is not a finger print, but a password or PIN (personal identification number) which is a numerical password. A PIN is required in most non-U.S. jurisdictions to use a credit card (e.g. certain exceptions such as the store cards of the retail chain Target), and even in the U.S., is required to use an automatic teller machine (ATM) card. The requirement that a PIN be used dramatically reduces the amount of fraud involving point of sale card purchase transactions. This is actually a good thing, because if hackers figure out your fingerprint, you can't easily change it, but if hackers figure out your PIN, you can just choose a new one.
But, because the amount of fraud involving point of sale card purchase transactions is fairly low, even when a PIN is not required (and some point of sale card transactions require neither a signature nor a PIN, just the presentation of the card itself), relative to loses from simple non-fraudulent inability to pay the amount of charges incurred, greater security has not been a high priority for credit card systems and they have further reduced the pressure from the public to increase security by indemnifying customers from any false charges applied to their accounts, a cost the used to be shared between merchants and the credit card system operators based on the circumstances and is not increasingly the responsibility of the merchant alone.
Also, in situations where there is a risk that a signature may be forged, for example, when someone is cashing a check at a grocery store, often a fingerprint is required in addition to the signature, with the signature evidencing the fact that there was a deliberate act intending to endorse the check and the fingerprint providing a reliable way to verify that the person signing the document was who they claimed to be (with a lie easily deciphered with law enforcement databases).
