All the money that's accumulating in the hands of the ultra rich has to go somewhere, and that somewhere is investment assets and investment assets are chosen based on going up in price and said going up in price needs not be for any reason other than that money is going into those assets in the expectation that the prices will go up (i.e. it's just a feedback loop of price increases attracting more buyers which in turn makes prices go up).
It's one big Ponzi Scheme and it will keep on going until there is no more money entering the scheme.
As it so happens we're living a long period of the wealth that was accumulated across society for decades being sucked out from the hands of the many into the hands of the few, and said few then keep putting more money into investment assets (what would be the point of buying yet another yacht) which is what keeps the whole price goes up because people buy -> people buy because price goes up loop going in so many asset categories, from housing and stocks to ultra speculative stuff like crypto.
This shit will only stop when it crashes and the longer it keeps going the harder it will crash.