The idea was that wood gets shabby a lot faster than metal.

So, if you make the butt oversized, there is a fair bit of "fair wear and tear" that can occur and be easily dealt with by basic woodworking skills.

The recoil is transferred via the buttplate, and it certainly doesn't exactly fit "the fleshy part of the shoulder" with precision, either.

The buttplate on a Lee Enfield is there to stop the wood getting smashed up either on the parade ground (not that the butt should EVER strike the ground with force if the movements are performed correctly), or on the battlefield.

It also allows the inclusion of the nifty little trap that retains your basic cleaning gear, not to forget, easy access for the armourer's big screwdriver.

When things go really pear-shaped, it also serves as the striking edge of a very expensive fighting stick / club.

Buttplates generally require a small amount of "fitting", but it's not a big job.

If the brass / mazakicon etc. plate overhangs the wood, it becomes a snag hazard for all manner of things, webbing, for starters.

If you look at the blunt end of a correctly presented specimen of Mr. Garandicon's rifle, you will see that the wood overhangs the fancy steel buttplate as well.