Many do use simple geometry. This causes endless headaches for people who are not cartographers, they don’t expect that. The good geospatial tools usually support spheroidal models but it is not the default, you have to know to explicitly make sure it uses that (many people assume that is the default).

An additional issue is that the spheroidal implementations have undergone very little optimization, perhaps because they are not the defaults. So when people figure out how to turn them on, performance is suddenly terrible. Now you have people that believe spheroidal implementations are terribly slow, when in reality they just used a pathologically slow implementation. Really good performance-engineered spheroidal implementations are much faster than people assume based on the performance of open source implementations.