I don't know for Stripe specifically, but I have been partner at some companies and can share some reasons from my experience:

- There are often limitations to who you can sell to, often in the form of requiring board approval before a sale. This is to ensure ownership of the company does not diverge too much from stakeholders.

- There are also often PUTs and CALLs in the shareholders agreement (e.g. the company can exercise a call on your shares if you leave, often with a discount if you're dismissed with reason, and you can exercise a put with some vesting scheme), along with a matching valuation formula. This often acts as a range for the sale of (often very illiquid) shares. Effectively, that formula is often really not interesting for pre-benefits startups, as its often some variation of "the average of the latest two previous years of dividends annualized for the next 5 years".

That’s a pity. As an investor I’d love to buy shares if Stripe, Epic, and other unlisted companies.