Probably because synths aren't as popular. Tons of people know a vintage guitar is something "special" because lots of people play guitar, and we've seen rock stars play guitar for a long time. Lots of kids grow up being really into any given guitar player and think "I want a guitar like them!" and, then you learn they play a 1956 Les Paul, and it's down the rabbit hole.

Gibson and Fender have the incentive to produce remakes. I'd guess to the average person, there's a lot less mystique surrounding a vintage synth, than a vintage guitar. It probably sounds like a 1996 Dell computer to them...

Some synth companies do, Moog being the most obvious, but they're more limited, and very expensive. I don't even wanna remember what those Moog modulars cost.

Also, guitar technology has only changed so much. Synths get cool new stuff fairly frequently, or the cost to make them gets considerably cheaper.