Although I think JimDandy has pretty much addressed the original question, the OP might be surprised to realize that the generalized appointment of justices with prior judicial and/or constitutional law experience is actually a fairly recent phenomenon. Many past SCOTUS justices, including many notable ones, were current or former legislators, state governors, prominent attorneys in fields largely unrelated to constitutional law, or advisers to the President who appointed them.
In the past, it was a basic and common presumption that a SCOTUS appointee was probably a political lackey to the President who appointed him*, albeit generally a well-educated and very intelligent lackey.
However, Congress generally deferred to the President on SCOTUS appointments, as it was presumed that the President was entitled to appoint anyone he chose, so long as that person didn't possess any glaring personality traits or history that would obviously disqualify him from the job.
IOW with many past SCOTUS appointments, there was no question as to whether or to what degree the nominee was biased, because everyone who understood the process knew that ALL the nominees were ALWAYS biased!
Yet... the SCOTUS continued to function and serve as a check and balance.
The difference is that the courts in the post-WWII era have generally taken a much more active role in overturning questionable legislation and policies. IMHO this has generally been a Good Thing for the average America citizen- particularly those with a darker skin tone- but it has created a good deal of controversy and increased Congressional oversight of the appointment process.
*I use "him" because there weren't any female SCOTUS justices in the time period I'm discussing.