Magento2 reindex from backend

Cron is all but required now. You will run into a lot of problems with your store if you do not have Magento's cron scheduler set up and running as intended, indexing just one of them.

Being unable to reindex from the admin panel is a design decision carried over from EE 1.13-1.14. It's unlikely to change back.

Under normal circumstances, with indexes all set to 'update on save', it shouldn't be necessary to reindex in the first place. They will all be partial-updated immediately with product changes.

There are free modules to add reindexing functionality to backend. For instance, Reindex by DigitalPianism