Windows Defender doesn't need to compete in the AV marketplace as it's bundled with every single copy of Windows, and also its maker is the same one who makes Windows, and therefore it is in MS's interests to make their AV light and unobtrusive (relatively). Other AV vendors compete fiercely with themselves and this leads to feature creep and bloat, as well as trying to grab the user's attention to sell their features and upgrades. All this leads to a subjectively poorer experience with non-MS AVs under Windows.
No, it's a question of security. AVs are huge and therefore significantly increase the attack surface. They also auto-update all the time which means your computer now talks to one more update server that can be compromised.
Windows Defender is relatively small, doesn't really have any features or fancy UI and updates come from the same servers that your OS updates come from (presumably). That's about as close as you can get to not making it worse by installing an AV.