i suspect it depends upon what you are looking at for ebikes. i’ve seen ebikes ranging from casual cruisers to emtb to cargo models and touring hybrids and they all have different intentions and tend to have different versions of seats and accessories…. i’d disagree that all ebikes are built for large and soft seats.
that being said, it seems “real” bikers are less interested in most models/varieties of ebikes and ebikes seem to attract an older crowd (perhaps with more disposable income)…
mindset of ebikers can be different as well, it’s not about pure human performance for most, it’s about comfortable efficiency and for some that may translate to more comfortable seating options as judged by the rider and not some predetermination of what an ebike seat should be. i’d say seat replacement/upgrade is among the top “improvements” a new owner makes, i did that with old mountain and drop bar bikes in my youth and ebikes today.
lastly, when pricing and components converge, i think seat selection is an area where a lot of bike companies can and do take shortcuts….