They became cost effective in a few years. The average price per integrated circuit dropped from $50 in 1962 to $2 in 1967. By then Moore's Law was established. At the end of the decade we had hundreds of transistors on a chip. At the early 70s we already had general-purpose microprocessors with thousands of transistors.

Quantum computing is being researched since the 1980s without a theoretical understanding whether they can be built at a practical scale.

As pointed out by others, they were very cost effective. And they replaced vacuum tubes, which were fragile. But, the most important thing is that they immediately solved a problem (ballistic trajectory calculations in missiles) and therefore had an industrial use which led to a virtuous cycle of improvement.

Quantum computers aren't useful; they're still searching for a use.

The two aren't really related. It's a bit like saying "we were able to build computers, surely we can also build <insert arbitrary tech thing>". Still waiting for that high-temperature superconductor.

As far as I understand it, the biggest problem is getting the manufacturing precision good enough. The more you scale, the more precise the manufacturing needs to be, which was never really a problem with microprocessors.

They might have stayed cost prohibitive for a lot longer if it weren't for the Apollo program buying up everything they could get

Than you. Somebody speaks with some common sense. All major breakthroughs start small and are not cost effectvie immediately. People should be way more enthused that this is possible rather than talking about all the negatives.