Real estate investors have to pay capital gains on properties they sell, do they never sell their properties?

That's not comparable. RE investors are buying with the intention of selling and realizing an after-tax profit. For most individual homeowners if they're selling, it's to buy another property. Say you own a $1M home, and when you sell you're going to face $100k in taxes based on the gains to date. Now if you're considering moving you can only afford a $900k home instead. That would present a disincentive to moving.