because we're still at the point where the browser is the everything app that works seamlessly across systems and users already understand how it functions and how websites generally work inside of it, supposedly on a layer with restricted access to the whole device. Apps require more trust or heavy usage for a user to warrant installing and giving additional exposure to the wider OS (at least I perceive it this way). At the same time, coding for the browser is basically unrestricted of gatekeepers and compilers and specific hardware, device shape or even coding skills to have something up and running everywhere.

tldr: less friction