Each funding round and new hire brings dilution.

The company issues new shares for the investors and new hires.

So, you might start with 100,000 shares and 10% of the company, but by IPO, there could be 100,000,000 shares, giving you 0.1% of the company.

(These numbers are completely made up, though if the company ipo’s for $1B, that’s $10/share which is a plausible price per share. Companies often grant additional shares to early employees.)