Why? They give you more early game options, keeping the meta flexible. Isn't that a good thing?
3x rifle start gets a little stale.
I don't like how they encourage picking a doctrine instantly. I can't be the only person who equips doctrines just so my opponent will be really fucked if he decides to use certain units, am I? (IE if he builds a kubel, I can shit out a jeep and instantly counter it.)
I'd much rather the jeep be buildable as a core american t0 unit than be locked behind a doctrine. In general, I'm really not a fan of calling in units that aren't late game tanks or battle groups. I prefer when they have to actually be built at a structure, using up time and a production slot.
The variation in American early game is in the decisions to buy upgrades or mix in RE/ambulances. A 2x riflemen 2x RE build has its strengths if you make good use of fighting positions. 3x Riflemen is just the current trend but it's far from the only viable option. (And past 50 fuel, Americans have a ton of great options in terms of tech and units.)
play with the unit before complaining about it
what's the biggest complaint US players have? No end game. What end game does mechanized give? jack shit.
You should be happy to see trucks, because if you can survive them (and you can) you know you won't be fighting easy eights or m1919 rifles
American T4 is one of the strongest in the game. Just because they don't have a heavy tank doesn't mean they don't have an endgame. Not to mention they have the best AT gun.