Because sometimes you need to manage third party scripts, pixels and tags. And sometimes Google Tag Manager is overkill. It may be...
- You don't want to call a developer every time a vendor script is changed.
- You only have a couple scripts that need to be added and don't need the full GTM experience.
- A client is going to take over managing their site and GTM is just another thing they have to learn/deal with.
- Some vendor scripts simply don't work through GTM.
- You're a rebel, Dottie.
Realistically GTM is a very powerful tool. If you need anything more complicated than, "run these tags on these pages", you should consider it.
Although, you can use Third Party to load GTM, just saying.
File an issue and pull when ready.