The laptop microphone will pick up whatever noise is in the room. It's easier to filter out background noise with a microphone that is closer to your mouth.
For the same reason you might have other external peripherals. A laptop, realistically, can only fit something so decent inside of it. The 2020+ MacBooks do have surprisingly OK internal mics, but that's about all they are: OK.
Maybe I'm just an extra-big baby about it all. But what I find a little annoying with the COVID-era of work-from-home and distance learning is how few people seem to care about audio. Even as a teenager on TeamSpeak, rather than getting a more expensive graphics card or whatever, I spent my money on an SM7B. Now it is more important than ever.
Maybe this isn't reasonable, but I feel like if not for yourself, to prevent "sorry, can you repeat that?" moments, you kind of owe it to the people who have to listen to you. Like as an autistic person, hearing a dozen people's overlapping background static, tinny compressed audio, etc, it really, truly slowly drives me nuts. I can't deal with that level of auditory sensory stuff all day. If someone has a bad microphone, I want out of the call ASAP.
A Shure SM58 will last you a lifetime, fit on your desk, cost <$100, and no one will ever complain about sound again.
People don't know how bad they sound. It's as simple as that I think, there is no easy monitor functionality in Teams and friends like the preview image for camera, so there is no urgency.
