* if everyone who sold donuts suddenly went into AI there'd be a huge profit opportunity in donuts - optimizing for profits would be to wait for the other donut sellers to switch into AI and rake in the cash.
* the cost of retooling constantly based on the latest profit fad would just make the toolmakers the main profit center, and the toolmakers would just use their own gear to take all the profits in abandoned markets.
* the constant shift of areas of business would be sub-optimal because most people entering it would know nothing of how to succeed in that field, it's not optimal for your company to be incompetent in an area with much competition.
* labor costs in the "only profitable field" would be through the roof as everyone scrambled to hire competent people - not an optimal way to maximize profit in a crowded industry (also, this compounds with the above point).
In fact this idea is so bad (and yet weirdly beleived by many) that every boom there's memes and jokes about how absurd it is that random companies from completely different industries are getting involved... as if they have a chance to compete against the established players. And even more jokes about how they predictably go out of business.