TON strives to be a decentralized system, as most blockchains do. That means that instead of relying on some "single superpower" it's better to have an "array of equal powers". And that means it wouldn't be good for the system if there emerged some "dominating super-validator" with an insane sum as its stake. It's better to have many separate validators with smaller sums.
That's why the system is tuned in such a way it encourages people to create more validators instead of putting all their money in a single one. It achieves that by curbing profits of validators after some limit (now it's about 900K but could possible change in the future).
When a validator has a stake between 300K and 900K, adding more Toncoins to the stake leads to more profits, so there is a motivation to do that. But after the ceiling level is reached, adding more money stops generating more profit. So there's not much sense to add money to a validator that has reached that limit. If someone has Toncoins and wants to stake them, it's much better to choose another validator that has not reached 900K yet so the investment will bring profits.
So there is no direct ban on adding "too much" Toncoins to a validator, they are not going to disappear or something like that. There's just not much sense to do that when there are more profitable options.