I think you make a fair argument, but it breaks down immediately under two separate but related counterarguments.

1. If companies don’t care what they’re valued at, it makes M&A infeasible. A business could never be sold.

2. Selling equity is done to raise cash. If no one cares what the business is valued at, equity is worthless, and 1. is infeasible.

That's not quite what I wrote. The larger companies don't care about you as the retail investor, but they of course care about valuation. But the valuation is often based on some belief / future predictions. Otherwise we wouldn't have large changes and following corrections. The value of the business does not change that much - the collective idea about the value, opportunity, chance of speculation, and other things influenced by the validation is what changes. Also no company would complain about having too high valuation as long as it's consistently too high.