Why doesn't the air force order more F-22's? I've come across several articles touting the F-22 as the most formidable air-to-air killing machine to date, such as this one:

https://nationalinterest.org/blog/buzz/f-22-nearly-unstoppable-air-force-has-too-few-them-32152

However, this article and others are also quick to point out there are too few of them.

It is not clear to me what the problem with restarting F-22 production is. The cited article appears to state that the USAF will focus on the sixth generation of aircraft, and integrate the F-22 into that.

Doesn't this seem like a huge gamble? When will sixth generation aircraft be ready in massive numbers?

There are new F-15's and F-16's being built, even today, and they still comprise the bulk of the entire USAF inventory. I don't buy the argument that they can simply "skip" the fifth generation in terms of air superiority fighters and wait for the sixth generation.

With the F-35 being in full production, is it really that hard to restart F-22 production as well? I'm aware that probably nothing at all on the F-35 can be just bolted on to an F-22 airframe -- but shouldn't advances in avionics on the F-35 (if any) only compliment a restarted F-22 production?

It seems at least plausible that the personnel and facilities involved in making F-35 would be able to be adapted to build new F-22's.

However, I'm sure it isn't as plausible as I imagine, and I'm just curious what are the reasons.
Aside from Juan's point about the F-22 being fairly outdated by now (it's an 80's era design), it is also not possible to just restart production of it. 

First, it would cost ~$10Bn to simply get to a place where you can produce the an F-22 again. That doesn't include the actual cost of a single plane, just the money required to re-equip factories, train workers, produce a new engine since the old one is end-of-life, etc. Source

This estimate is assuming that the project was left off in the state Lockheed claimed it was. There is reason to believe that some of the tooling for the F-22 no longer exists at all. Source

Finally, the F-35 program has, in my opinion, shown that there is not so much to be saved by re-using components if the overall process is not efficient.