Why does high cadence work feel so much harder?

Totally Normal. You are teaching your muscles and neuromuscular system something new. Just like running, you will get better at that cadence and it will be easier. I would caution against striving for perfection of that 180 number. Each person is different. I have found that it is best to get out of the 160's. That is where poor running form occurs. That is the main thing.

i got this...

Probably because you’re slow. And you don’t have to start this thread every few days. Just bump your old one.

It’s a bot

BS. The most efficient stride rate is the one you don't have to think about.

In his/her/they’s defense instagram fitness pages ALWAYS divulge blanket remedies based on their social media physiology degrees. And people follow it like dogma as they gush and wonder why beautiful, affluent, white people can be so successful.

I don't necessarily agree with this. I think the body will generally do what it thinks is "easiest", not what is necessarily best for running a marathon safely. I think a huge reason why newer runners get injuries is because of their ridiculously low cadences (often in the 150s) and long strides, which is what their "natural stride". Might work for running a few hundred meters here and there but it's not going to hold up when moving on to serious running.

Same thing if you put a heavy barbell on the ground and ask someone to lift it up, they're not going to properly deadlift it up with good form, they're going to use sh*t form, throw their body backwards, bend their necks, etc. They can get away with that form for a single deadlift, but not if they are looking to get serious about lifting, it's an injury waiting to happen.