It's similar problem to how Chromebooks have still not really displaced Windows laptops except at the very low end of the market.
Although the significance of this seems lower now than at any time I can remember as more happens in a browser than ever. Windows 10 apparently is fully ARM compatible with a built-in x86 emulation layer, so perhaps it could work.
Also other companies don’t have access to apple’s arm chips with great single core performance.
They already do, but they aren't as popular or as capable for two reasons:
The first is that only Apple can take advantage of any speed or power management improvements that a switch to a different CPU might provide, and that's because software development for Windows is very much not slaved to the App Store in terms of application development. That wasn't through lack of trying by Microsoft in the early 2010s (Metro/UWP and the disaster that was Windows 8/8.1), but as a consequence of that failure Windows applications aren't developed with a mobile-first paradigm or cross-platform support, so their ability to be managed by the OS is more limited.
The second (which is also an implication of the first) is that the only time ARM can actually outperform x86 designs is when a single-digit-watt power envelope is required. In Apple's case, it's required because they refuse to implement a proper thermal management solution in their laptops, and as such using processors that can reach 4 GHz for about 5 seconds until they hit 100C and have to aggressively throttle back are a waste (and their fans spin up to 100%, and excessively heat up the keyboard and the base of the laptop).
By going to a processor type that will ultimately be significantly slower but much more consistent in terms of performance, it's possible to both use alternate cooling solutions (like fully passive cooling) and provide a much more consistent user experience. And provided that the GPU is any better than Iris IGP, and that video encoding is otherwise accelerated (i.e. Quick Sync), professional (non-developer) users will still notice an upgrade.
And sure, while developers will lose in terms of raw performance, Apple knows that developers that go out of their way to use their products are more than willing to make compromises and concessions to use their ecosystem. On the other hand, those who are concerned about performance don't (and never) buy Macbooks.
I read in another thread that volume is what propelled x86s popularity. Intel’s difficulty with 10nm has created an opening for other fabs and other architectures to catch up. If ARM can be shown to work as a desktop processor, why wouldn’t HP, Dell, Asus and the rest make ARM laptops?