Also many events have a stated capacity, I assume the website blocks entries once this limit is reached ...
I don't think it does, but it would be a nice feature to have
In effect an organiser can close an event once an event is maxed out.
The problem some have is whether to overbook in the knowledge that a proportion will not show up on the day - even when the weather is fair! In PBP years this is really hard to predict (especially later in each qualifying window).
And on popular rides a few days warning of a DNS means that the place can very often be taken by someone else.
On London Wales London I'm deliberately offering a partial refund to people who signed up if they drop out early; I find that when we get to the end of January people's plans have changed or fitness isn't coming in at quite the right pace so I normally expect to start offering waiting list places to people.
The Bare Bones 400 won't have a limit on numbers - it's near the end of the 400 qualifying window so no one needs to miss out because of a shortages of 400's.