It's simple: we don't allow it. They force it.

By holding wealth they hold power, by holding power they can use coercion and violence to ensure they keep power

So that unless a large enough majority would choose to fight back, which is a huge risk that most wouldn't want to take, they can just fend off all attempts to remove their power with very little difficulty

And you may say "but then why doesn't a majority form to fight back?"

• fear of repurcussion - what if you lose the fight? They will destroy everything you care about in retaliation
• corruption of state - the state itself is willing to prop them up due to those with wealth using wealth as a tool to rot it from the inside out
• class traitors - those in power create a mythology that anyone could be like them, it just takes hard work, in so creating people who have no power that are willing to fight for them cause they think it will give them power (it won't)

If things get bad enough, then all those bullet points will not outweigh the amount of people willing to fight back, it's happened time and again. But in almost every case it's gotten MUCH MUCH MUCH WORSE than our already shitty situation before there was enough will to fight.