Why is not a good idea to have the domain and hosting with the same provider?
You may lose both your domain + hosting at once without heads up for starters.
Not saying it will happen too all providers but, some may, hold your hosting or domain as a hostage. (This has happened to many people in the forums, search it up)
Also, some hosting providers will actually "Own" the domain name, and is renting it to you, which means, basically, its a shit show. There nothing more to say to it
Sense of stability, what happens if you choose a smaller-based company. It is always recommended to choose a big brand for domain registering.
Companies fall and disappear daily, if you lose hosting, sure, that's ok, you may have backups you can restore.
If you also have domain with them, you basically lost your domain as well.
Just a few reasons why, and more than enough to not do it.