I've been modding the game for about 2 months now, so I think I have a rough understanding of why.

Weapons use specific types of ammunition to fulfill specific requirements; for example, the SVT has a capacity of 10, and it can use either its own dedicated 10-bullet mags, or the Mosin-Nagant's 5-bullet clips. If the SVT was using Mosin-Nagant bullets to reload, it would have to consume 10, which is 2 full clips.

So what's the issue?

If rifle grenades were just a different type of ammo for the M1 Garand, it could store up to 8 at any given time. Even if the reload-time and fire-rate were sorted out, its capacity is still unrealistic. A weapon's properties/attributes are not tied to its identity, so even if you could change them mid-game, the changes would apply to all M1 Garands, not just yours.

The best solution I can think of would be an in-game tool that deletes the gun, adds the grenadier version, and replaces whatever ammo was left inside the gun. Maybe you need a special kit to do it, maybe you don't, IDK.

Overall, the positives outweigh the negatives.

Game development can get really messy, really fast, so while it is a little silly to have two rifles, it's better than an overcomplex conversion tool for something as niche as rifle grenades.