Stock dilution doesn't work like that. If a seed investor invests for 5% at a $10mil valuation, and the company goes 10x (ie. a valuation of $100mil), if the company now raises a $100mil Series K, that means the Series K investor owns 50% of the company, and the seed investor got diluted down to 2.5%. However, the new valuation of the company is now $200mil with the cash that the new investor brought in, effectively making the seed investor's investment worth the same.

It's a smaller piece of a bigger pie.

To answer your question, the right question to ask is why go public when you can remain private? Public means more paperwork, more legalese, more scrutiny, and less control for the founder, and all of that only to get a bit more liquidity for your stock. If you can remain private, there really isn't much of a reason to not do that.