The issue is not why such support is not included or whose fault it is, although Apple's failure to build in full mpouse supports puts a lot of the blame square on them IMHO; but the lack of it make it a lot more difficult for apps to be true desktop replacements since simple things are much harder to do; especially in productivity apps where a mouse makes it alot easier to make fine adjustments.
Full mouse support isn't going to get you to a full desktop replacement device. You're going to be asking for iPads larger than 13". You're going to be asking for more USB-C ports. You're going to be asking for the ability for apps to be fully capable of running the background (which means you're going to want more ram). Some are going to ask for function keys as professional apps like Xcode really need them. You're going to ask for being able to have multiple apps running with several windows on the screen at once. We already have a device that does all of this really really good. That's a Mac. There's no point in setting the course for the iPad to become the Mac when the Mac can do all of this really really good.
Let the iPad be better than the Mac in some areas and let the Mac be better than the iPad in other areas. Current technology doesn't allow either of the device to be better in all aspects. So they currently coexist.