there are people who start w. a seakayak, I know a few. I got mine four months into paddling.
But they are in a minority. Why that is, I think, has a lot to do with how kayaks are marketed and the general perception that just about anyone can get in a recreational boat and go paddle. In contrast many view a seakayak as inherently too tippy and the smaller, skirted cockpit induces claustrophobia.
A lot of people getting into the sport like a SOT or rec boat as they are easy and comfortable to use. Nothing wrong w. that. If their interests and paddling venues get more diverse and/or they really get into the sport, they may choose to paddle different boats. Nothing wrong with that either.
Whatever boat you start with, it's important that you, the paddler, have the skills to deal w. the range of conditions in the waters you normally paddle. And part of the skillset is knowing the right boat for the right conditions... and when no boat is suitable (e.g. stay home)
If Person A likes flat water ponds or small gentle streams in good weather the demands on the boat and paddler are obviously less. Person B who wants to kayak large open water, play in surf, go rock gardening, or do Class IV rapids, and will go out in cold water, or days when a marine warning is out, is better served with an entirely different skillset. If their judgment is developing along w. their skills they will probably conclude they need something other than a rec boat.
Plus it's just plain fun to have different boats if one is so inclined.