Wow, why would they limit they?
Its like purposely crippling a team for the sake of a single mistake at the start of a season.
Cost. The gearbox mounting points are standardized so in theory any team could buy a gearbox off the shelf and bolt it to any engine.
It also made a bit of a mockery of the 5 races per gearbox rule if the teams were able to strip them down to replace the ratios every weekend.
And it encourages the teams to manipulate the ERS settings to manage the tourque for different tracks rather than just changing the gear ratios, which fits with how the technology would be used in road cars.