In a way they do because for years the updates come primarily through the Play Store as most of the functionality has been moved out of the firmware and into updateable components. Also a big effort reaching into Qualcom itself who provided drivers that abstract hardware meaning that changes deep in the system can be made via the Play Store as well.

This is why vendors are able to offer longer term support, the work that they have to do has been greatly reduced and simplified.

A verdor update will still offer some refresh of the UI they provide but the core phone is basically keep up to date with the need of there input.

When a vender update does come it requires a sign off from you countries ISP before it is sent to your phone via the update process.