The charter we signed "forced" us, as it did all of these countries that signed it. You could ask why they signed at all if they weren't committed to spending 2% GDP on it as specified and instead rely on US money to handle their defense.
It doesn't force a 2% spend and it's not in the NATO charter. The 2% spend was an aspirational goal which countries pledged to in 2014 and the target date for that spend is something like 2024
It's very difficult for Germany to efficiently spend that amount of money given their other treaty commitments namely the 1990 "Treaty on the Final Settlement with Respect to Germany" which limits the size of their men under arms.