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).