Yes I know those hybrids exist, but unfortunately there aren't nearly enough of them to fill all the positions. Not only that, but I'd argue that a lot of them who are hybrids, you're not going to get their best work from both areas if they are expected to deliver from both disciplines on projects. Not on the deadlines that employers will set at least.
And I suspect a similar problem exists in actual building architecture. Yeah some building designers are good at accomodating for all the variables of planning codes, but they aren't going to be (nearly as often) fantastic at aesthetics that somebody who works outside those constraints might be capable of. There's a lot of value in designing first, then put those styles of thinking together (designers and architects) separate people and from that point later in the process of creation you start solving the challenges like plumbing codes, metallurgy afterwards. Part of this is me complaining, but it's my attempt at experience of always being a crossover person and how you never really feel like you're honing a craft, you just feel like you're being left behind and not properly cultivating what you enjoy most.