You don't know what future funding rounds there'll be. Maybe you raise a seed investment and then decide to sell the company two years later. Maybe you'll raise 5 more rounds and IPO.

Also, if you sell your own shares the money goes to you, not the company. I guess you could then loan the money to the company, but you'd still be liable for tax.

Going from owning 1% of a 1M company to owning 0.5% of a 2M company isn't unfair. The extra value comes directly from the new shareholders investing their money.

Employees believing their ownership percentage will not change is a problem though, and founders should not suggest that to be the case.