If the velcro is under the seat where it can get wet, compressed, contaminated or abraded - that could be a reason. I have foam seats in 3 of my yaks: one well jammed [to the sides and deck], one the same and could/should be better jammed, and one for years that has been velcroed:

That seat is about 4" high out to the sides. The kayak sides are approx vertical, the seat at the sides is approx horizontal. I bought some 4" wide velcro and cut four 6" lengths, two of which i cut in half longitudinally to make 2" wide velcro strips. I glued one of those on each seat side with the long length touching the kayak side and one on each kayak side directly up from that horizontal velcroed surface.

I then folded 6" longitudinal "L" shapes with the remaining 2 pieces of 4" wide velcro, placed the seat and stuck the 'L' shape over both the horizontal and vertical 2" wide glued on velcros and now can adjust that seat in place back and forth several inches if necessary.

Any saltwater drains right away, no dirt to contaminate and reduce velcro effectiveness, and the toughest thing is to remember to unvelcro from the kayak side when seatn moving or I'll rip the foam right from the velcro. If to do it again, I'd urethane glue down a strip of ply on the foam first before the velcro.

And on 2nd thought if the foam doesn't reach the sides, make 2 ply extensions and 'L' velcro those to the kayak sides - might get some storage too, as wall as the adjustability.
6" of two in wide velcro on each side really sticks!