Only Samsung and Google will have firmware updates, the others would be 3 yrs per Qualcomm SoC support. Samsung only have long term support on their flagship models.
Out of the two I would pick a Pixel over Samsung phone. You don’t have to then deal with bixby or any of the crapware that Samsung includes, like facebook as a system app etc. It also means GrapheneOS is an option.
They don’t because the devices have weaker hardware backed security than the Pixel, and no vendor support for firmware updates beyond what Qualcomm offers.