Civil forfeiture typically pits the government against property, not the government against an individual, and in the US (also anywhere else), only people have rights: property has no rights. The first relevant instance to reach the Supreme Court was an early case, The Palmyra (25 US 1), where a ship was confiscated because it had been used in privateering against the US. Although the owner had been convicted of nothing, the court still allowed the confiscation since
The thing is here primarily considered as the offender, or rather the
offense is attached primarily to the thing, and this whether the
offense be malum prohibitum or malum in se
The defendant here is the ship itself, not the owner of the ship (known as in rem proceeding). The case law is replete with cases like "US v. $500,000", and such forfeitures have long been part of the arsenal to be used against wrong-doing, a law authorizing this having been one of the first acts of the US Congress, especially applicable to ships and cargo involved in piracy. The courts have repeatedly allowed such confiscation, at least in certain circumstances.
In one more recent case, Van Oster v. Kansas, 272 U.S. 465, the court stated that
It has long been settled that statutory forfeitures of property
intrusted by the innocent owner or lienor to another who uses it in
violation of the revenue laws of the United States is not a violation
of the due process clause of the Fifth Amendment
followed by a huge list of citations. Because of the doctrine stare decisis (courts respect prior rulings in similar cases), once a pattern is deemed constitutionally acceptable, it is very hard change that interpretation of the law.
It should be noted that J. Thomas recently hinted in a dissent that there is a Due Process problem with civil forfeiture, raising strict scrutiny type objections to civil forfeiture, specifically that forfeiture was typically more narrowly applied – limited to customs and piracy, and
justified by necessity, because the party responsible for the crime
was frequently located overseas and thus beyond the personal
jurisdiction of United States courts
also observing that
founding-era precedents do not support the use of forfeiture
against purely domestic offenses where the owner is plainly
within the personal jurisdiction of both state and federal
Essentially, the reason why civil forfeiture (in its current revenue-generating incarnation) hasn't been ruled unconstitutional is that a persuasive argument has not been made to that effect. The court can't just decide this on its own, and in the most recent case, the constitutional issue was only raised at the Supreme Court, not in the Texas appellate court where it should have been raised.
The most significant problem is that it isn't clear what constitutes "due process of law". There is, in all jurisdictions, some kind of legal process (see this report for an overview of the kinds of process involved). The standard of proof that allows forfeiture is pretty low (being a civil case, it is not "beyond a reasonable doubt"), and there are various impediments discouraging anyone from suing to prevent forfeiture (which until 2000 included a substantial bond requirement and the threat of having to pay the government's attorney's fees if you lose). Since there is a process whereby forfeiture can be challenged, a more sophisticated argument is needed, based on Due Process.