Either it's an established vendor with designs and fabs or it's a newcomer that needs to invest a massive pile of cash in designs and fabs. Neither are cheap.
Seems kinda hard to believe at this point, no?
There have been SEVERAL crashes that have wiped out the market and it's the reason there are so few players, the rest of them went bankrupt after periods of over-expansion. (in the 80s caused by Japan, in 1997, in 2001ish after the dotcom bust)
You're even calling it a bubble so it's not exactly "hard to believe" it will pop.
Insufficient law enforcement. The same memory manufacturers already broke antimonopoly laws in the past, pleaded guilty. Apparently the fines were too small for these companies to care, and the people responsible were promoted instead of being punished. More information: https://en.wikipedia.org/wiki/DRAM_price_fixing_scandal
In this context, it takes spending enormous piles of money over the course of at least several years to spin up new semiconductor production.
We do need more capacity to keep up with AI datacenters' usage, yes.
But adding long-term capacity years down the road for a thing that some folks seem to confidently think is a bubble that can pop at any time is risky. And (because capitalism), we have to manage carefully balance our risks and rewards in order to maximize our odds of success.
If there is no bubble and demand stays high long-term, then the payoff for that risk is potentially enormous.
If there is a bubble and it bursts, then the cost of that risk is potentially devastating.
(Capitalism works most-predictably when cheating is possible, such as with Biff's use of the time machine in Back to the Future II. But without cheats, it's always a gamble.)