I've found it's not necessarily "hard", but the strategies that worked in school don't work after graduating. At school friendships just kinda happen - afterwards, you need to work at it. I struggled for a while, in my first two years after uni and leading up to Covid. What helped me was:

* Joining a very social team in a very social company. This was in 2019, and even though they went bankrupt and we all found other jobs, a few of us still hang out regularly.

* D&D. I'd played a bit at Uni, and once lockdowns subsided I convinced my girlfriend to come along to a local club with me. Now we're both running full-blown campaigns!

* Volunteering. I joined a local food-aid group, and it's been fantastic to have something with a bit more meaning behind it.

Once you've got a few good friends, it becomes a lot easier to make others.