Because of the frequency of the updates and the depth of them. It's not just fixing bugs, it's improving efficiency and adding new features.
It is true that Tesla's OTA system is the most mature, they've had 13+ years to make it so, which isn't surprising. Also, Tesla's model is to never require a dealer visit.
But there's a big cost to this: Tesla has had to re-engineer traditional taken-for-granted features from scratch and has been slow to do so and also buggy in a number of areas - such as auto-wipers, auto-headlights - because they started from scratch and wanted to cut the cost of dedicated and proven hardware for these systems.
Other makers are sort of part way there: e.g. there'll do a bunch of updates OTA, mainly infotainment related and adding easily-implemented features in software. Some also issue updates to core driving systems. Then there will also be dealer-fit-only updates when they are less confident about the results (which is true of Renault, VW and others).
MG has been absolutely nowhere, despite multiple generations of promises of OTA across the MG5, MG4, MG Cyberster and MGS5. Still not a "sausage" to be seen anywhere.
So MG has fix this to be taken seriously going forward.
IM cars are different technology entirely, and hopefully they've thought more about OTA and made it much easier and will launch OTA updates soon.
BUT: until they do, I remain firmly skeptical. I've heard it all before - just like the "semi-solid state battery" - which is also nowhere to be seen and promised multiple times.