So you want snipers to be able to garrison vehicles again? I think that's very sad. It was one of the worst features of this game.
To answer your question, they shouldn't because when they can, the play has the option to build almost exclusively one unit. One fun thing about CoH is that spamming comes with severe drawbacks usually. The drawbacks of spamming Maxims re not sever enough to stop people doing it and playing against Maxim spam is borrrrrrringgggg. Fun > "immersion".
You'll be lucky to catch a maxim capping alone that doesn't have another maxim behind it. Check ImperialDanes Jellydonut shoutcast for an example.
Yes, I want, actually. I don´t have much use for it myself, but still makes one wonder why can´t a 2-man squad hop into car where 6-man conscript squad can. Now I´m totally fine with snipers not being able to shoot from there, that was bs both gameplay and realism-wise.
About maxims, I do agree playing against spammers is boring. The problem is the spamming itself, not their ability to cap. Inventing such limitation would only harm those of us who don´t spam these units in the first place. Btw, you spesifically mentioned lone capping maxim in your post, not one supported with another, but that´s enough about it.