Because real-time malware scanning causes way more trouble than it is worth. It adds latency to every file open on the off chance that it might contain some kind of identifiably malicious code. That might be mildly useful if I had any faith it could actually detect the majority of malware, but I don't. In fact, the only things I've ever seen it identify reliably are any kind of piracy or forensics tools.
Personally, I am not a fan of Windows Defender in general, but disabling it completely renders other services (e.g. Windows Store) unusable. That's why I only disable the real-time scanning component.
There are two reasons for doing this:
Firstly, it has been shown that the real-time scanning features causes issues with file access latency. It's apparently quite noticeable when using WSL. On a laptop there might also be a noticeable improvement in battery life, but I don't have any data to back that up.
Secondly, every now and then the real-time scanning triggers on some file, removing/quarantining it without asking me first. This often happens, for instance, for video game cracks that I'd like to analyze / revers-engineer [^1]. So, for me, it's a lot of false positives and so far I haven't encountered a single case where Windows Defender actually protected me from something. At least that I know of.
^1: It's a hobby, I don't build DRM, don't worry. The games I work on typically release DRM-free.