That's because Vanguard is a buy and hold philosophy. They have previously stated that they designed their site for the bulk of the users. Those users log in just to check their balances in the 5 or fewer funds/ETFs that they hold. So instead of building something that would be reasonably easy for everyone, they built something that was easy for their primary (aging) user base. It's the same sort of mentality how they don't allow inverse ETFs, they aren't going to offer any crypto related stuff, etc.
At some point you need to realize your gains—perhaps to rebalance a portfolio, or to pay for an expense, or because you’re retiring and now it’s time to enjoy the profits of your buying and holding. And a lot of people elect specific-lot cost basis accounting to minimize tax burden.
