While markets are used as a form of gambling, they also have a prosocial purpose, namely to allocate capital which improves the economy and society at large. Market manipulation increases market volatility and hence hurts efficient capital allocation, without some other benefit for the market. Besides that, it requires large amounts of capital to do, and hence can be effectively regulated.

Most of the stock market activity is secondary trading, which has little to do with allocating capital. Trading existing shares just redistributes ownership.

The stock market has a number of essential components without which it could not exist, all of which support it's role in allocating capital. If there was no secondary market to sell their stock to, someome might not invest in the first place. If there were or were not market manipulators cheating others out of money, that would make no difference as to whether someone invested in a company.

Secondary trading enables risk transfer, which is important for allocating capital.

Your first sentence is somewhat accurate, your second is not at all, specifically "just".

I didn't say stock markets don't serve a useful purpose. Regardless, my original point is simply that traders should focus on fundamental value, not try to guess what others are thinking. If they choose to play that game and get burned, that should be on them. There's no real necessity to regulate "market manipulation".