I don't understand why there was no standardization effort equivalent to ATX for the classic IBM ThinkPad-era designs.

A standardized form-factor allows easy replacement of motherboard, CPUs, screens, keyboards. Desktop PCs still use ATX cases, ATX power supplies, ATX motherboards.

Desktop PCs can be upgraded while keeping the monitor, keyboard and mouse: the entirety of a high-end gaming PC upgrade can consists of a new motherboard, CPU, RAM and GPU.

There's little reason laptops couldn't have went down the standardization path in the late 1990s. The chassis of a 1998 IBM ThinkPad should be able to components from 2019 without much effort. Just like an ATX desktop case.