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TL;DR
IMO USF has many options because its design is to have an ideal indirect fire for early/mid/late game
Doctrinal indirect fire tools add even more stages (MTHT, PRIEST)
USF used to have desing that was about having the best mainline infatry, good support weapons, good light vehicles, good tanks and hard hitting but fragile TD. One of their "weakness" was the limited access to those support weapons.
Most of the original design has gone out the window so if anyone actually knows what is the current design of the USF I would be more than happy to hear it.
Designing USF as having "ideal indirect support for all stages of the game" would mean that they would be able to beat Ostheer in a static play and that imo would cause a need to redesign Ostheer (or create other weakness in USF line up) so imo is not a suitable design for the USF faction.
If one looks at sheer number of USF weapon used in WC2019 (136 support weapons in 49 games 2.77 per game) compare to ostheer (105 support weapons in 40 games 2.63) one will start to understand the current nature of the problem.
In sort the gap in the superiority of Ostheer support over USF has become smaller and the creates problems for them. Grenadier can not match riflemen, Grenadier and support weapons can not match up riflemen and support weapons so people turned to alternative option like AsG and Ostruppen.