Why shouldn't individual stocks make up most of my portfolio if the goal is to accumulate wealth?

Disclaimer: I've only been investing for about 3 years so perhaps there's something obvious I'm missing here.

Outside of WSB, the notion that one could pick stocks is mostly frowned upon. Most of the advice I've seen is that 80% of the portfolio should be broad market ETFs, with the other 20% for individual stocks if you so wish, and that too never in registered accounts like Roth IRA/401K/TFSA (Canada).

Is this not counter-intuitive if the goal is to maximize wealth? I understand that for moonshots, a non-registered account is ideal because I can harvest losses and offset gains. But if there is conviction that a stock is poised to grow or at least maintain it's position, why would I not allocate most of the space from a tax sheltered account to these picks?

At the moment, about 75% of my portfolio is split between RKLB, SNDK, and MU, with the rest split between high conviction picks like ASTS, SLS, and MRVL. Traditional wisdom says I should trim my winners to rotate into ETFs, but what's the point of cutting my winners only to get a lower ROI on my gains?

Is this remnant advice from when having access to the markets was not as easy as it is now, so people weren't as flexible when it came to buying/selling? Or is this just advice created for financial advisors to limit their culpability?