That's because you should be using scribed joints rather than mitres.

Dave

Working on building sites, I notice that the some if not all of the
chippy's use Gripfill or similar as well as a nail gun to fix their
skirtings(and anything else come to that)! Neither do they seem to
scribe the mitres as most of the second fix "timber" is MDF and not as
prone to shrinkage as real wood. Now I know that a lot of builders still
use proper wood but not in many of the mass produced houses of lower
value. My own lounge/hall/dining room skirtings are all adhered to the
walls nor have been the mitres been scribed and all are still as tight
as the day they were fitted. People may shout and say that you should do
it this way or that because that is the way it's always been done but
you should also think why. If modern materials are much more stable than
timber and don't move what's the point in scribing to compensate for
movement that won't happen?

Grumble