I think the word harder is the problem

It's different.

They are just two different types of competition.

For example, here are the past 20 years of English UCL champions and their EPL table position:

Liverpool (2019) - 2nd
Chelsea (2012) - 6th
Man U (2008) - 2nd
Liverpool (2005) - 5th
Man U (1999) - 1st

Only once did the English Champion also win the UCL in the past 20 years.

If you compare that to the other winners, it's a different story.

Again, I don't think harder is the right word. They are different types of competitions, and the English champion doesn't always make for the best UCL team. But I'm not sure "harder" is the right word. The EPL is a slog that rewards depth more than anything else. That's not the same as the UCL.