Two concepts at play here: 1. ET is a team game. 2. No class is perfect.
You might as well be asking why medics cannot give themselves ammo or FOs cannot heal themselves. The point is that no one class can do everything. You need a balanced and orchestrated team. The latter is more difficult to achieve in pub play on a regular basis. That is why people tend to find a server that has a set of ‘regulars’ that play there and work together. By working together and playing their roles, rather than rushing out as individuals, they can achieve/defend the objective well.
As to the particular suggestion that higher level MG42 soldiers get more ammo, I don’t think this is unreasonable. At least one extra belt would make sense at higher levels. Medics gain an extra magazine for their SMGs, don’t they? On the other hand, I would expect no changes of this sort, so its really an empty request. You might be better served aiming the request at the modding community (i.e. bani and shrub) who may be able to implement something of this sort.
Dawg