Why should poker sites do that? Each table occupied takes some server usage and is not available for rake of other real players.
I think late registration is a fair strategy to limit your time played and maximize possible return per hour played.
You can easily register at the latest blind round of a tourney if you know how to play short stack. Either you double or triple up soon or it's up to the next one. If you do so, you usually have more than most other players who played an hour or more already.
I always late register with regular MTTs, if available, and I'm not a bot, or am I? 🤖
In case it's a PKO or turbo/hyper speed it makes less sense to do that and indeed one can notice that in the very early stages of the tournament, there's very few players because stacks are so deep and there's no big action happening.