Necessity of tenure for lifetime academic research
The reason is simply that possessing a tenure-track faculty position (and then obtaining tenure) is the primary performance indicator for long-term careers at most institutes where research is the primary responsibility.

There are some exceptions - some universities offer a research professor position: you pay your salary from grants you secure, and as long as you do that you can continue working there. 

You can also work as a lab manager/research scientist in a university, which is a term contract that is extended based on merit.

Some private research institutes (Google/Facebook/Amazon etc.) offer research positions (that are highly competitive and well-paid), but there are also many other private research laboratories that offer a similar deal to what you describe (though in that case your research needs to be focused on what these institutes care about, and is not entirely up to you). 

I'd also mention that tenured professors are still expected to be productive and get grants. Though it is more difficult to fire them, becoming completely inactive for no reason is probably not going to be good for your long-term career. In other words, to some extent tenure track faculty face the same agreement, especially when they're not tenured yet.