Mostly because updating 1 word on a database, has such a huge impact.
So for example, the council changes the name, that then means the stop flag (if it includes the stop name, which most LAs do) and bus stop timetable need replacing. Any real time information displays at the stops would likely also need updating with the new stop name. These are normally within the councils control, but not always. It also means that operators need to update various of their systems so that data is consistent, Not just making sure the stop name fits into the next stop display (most of these have character counts which are lower than the max character count on Naptan), but also get any announcements updated.
And all of this, relies upon the local authority being willing to update the database. It's taking some authorities months, if not years from first being contacted, to actually updating coordinates so that journey planners tell people the correct place to get on the bus and so announcements trigger at the right times etc.
Leicester City Council who installed new shelters with big boards for the top at the train station a few years ago, but still 3+ years on, refuse to update the stop name to match the new stop name on the flag (and match the other 3 stops in the row). They claim the stop wasn't renamed, despite physical evidence being there for 3+ years, and operators all manually overriding their systems to show the new stop name. Council says no, and as they are the gatekeeper of Naptan, there is nothing anyone can do to get it sorted, you just need to wait for the council to employ new staff in the offices eventually and hope they apply more sense to the situation.
So while I fully agree, it SHOULD be much easier and quicker to changing a stop name, sadly too many local authorities are ignoring Naptan and don't understand the importance of it and importance of having it all correct and upto date. Even if authorities are good at updating the database though, they then need to look at any costs to update roadside things. Then they need to be speaking to operators and rely on them to update all of their systems properly or you end up like Stagecoach and just announcing/showing incorrect details because they are using years old versions of Naptan (and let's be honest, operators already can't be bothered now to set it up properly now. What hope is there for them to maintain these systems long term?).