Autonomous weapons make up for their lack of soul wavelength with physical power and skill. The whole reason the meister/weapon relationship is so important is because they rely on each other. Meister produces the wavelength, weapon amplifies it. That is why a weapon cannot produce their own. Because they amplify wavelength they are given.

Anime ran out of budget or something. Not enough time to get to the end of the arc, etc.

That's beside the point. Manga came first. Manga is canon. Maka also being a weapon is stupid and I hate it. Bravery Wavelength is just some made-up bullshit to bring everything to a close quickly. The fight against Asura lasts sooooo much longer.