That is because you really have no idea why tenure exists. It protects academic freedom - a faculty member cannot be fired because of something they said or published. That not only preserves the integrity and quality of research, it also allow universities to compete with industry.
The attraction of being able to do your own research, rather than focus on what your corporation wants, is huge. Universities cannot compete with industry in salaries and bonuses, so the only thing that they can offer is something like tenure. Without tenure, only people who industry is not willing to pay much will become faculty.
So, if you get rid of tenure, your kids, your doctors, your nurses, your IT people, the people who develop computers, cars, the electric grid, your house, roads, your medicines, etc, will all be trained by the rejects of industry. Would you think THAT was a good idea?
Tenure protects the quality of teaching and of research, as well as the quality of the teachers and the researchers. Do you want to know what it looks like when you get rid of tenure? Take a long hard look at the for-profit “universities” - their abysmal quality of teaching, their lack of standards, their minuscule graduation rates, their predatory practices, etc. Is this what you would like to see as the sort of place you would send your kid? the sort of place where your doctor would train?
As for “fair” - how is tenure less fair than the hiring and firing processes anywhere else? It is, in fact, the typical hiring practices in the USA that are egregiously unfair.
FYI - “tenure” does not mean “now they can’t be fired”. A tenured faculty member can be fired if they do not perform their job, if they violate the rules and laws in the faculty handbook, if the university declares financial exigency, or if their unit is dissolved. They are protected from being fired because of their politics, their publications, or because they called the provost and idiot in public. Their contract is also open ended, meaning that they do not need to be rehired every year - it’s automatic.