1) The hardware quality of other OEMs is bad too. Lenovo has really gone downhill. I sold my maxed-out X1 Carbon 7th Gen, which in less than a year: a) came with an LTE card that never worked for more than an hour at a time; b) had the display bezel start peeling off (it's a sticker!); c) developed a bunch of dead pixels; d) started randomly locking up; e) had weird issues with power usage, where some service related to the touchpad would go crazy and start using all the CPU.
2) Other laptop makers make design choices and trade-offs that I find unacceptable. For me, the 16" MBP is the perfect balance of power, weight, and battery life. It compromises by using a 3K screen instead of a 4K screen, and gets 11+ hours of light usage. Alternatives like the XPS 15 or Lenovo X1 Extreme force you to choose between an FHD panel and good battery life, or a 4K panel and substantially shorter battery life. The Surface Laptop 15" uses a 15W ultra-mobile processor in a machine that runs $2,800 when configured with 32GB of RAM, and has a battery half the size of the one in the MBP 16". Many 15" laptop vendors include a numeric keypad, which is a deal-breaker for me because it forces you to type at an angle to keep your hands centered in front of you when in the home position.
3) Touchpad and high-DPI scaling.