Certain unnamed companies cough~google~ doesn’t like to trust Let Encrypt its definitely not an abuse of an illegal monopoly they have good reasons I promise.
But the whole point behind using a signed certificate is that other people can look at you and immediately know you are who you say you are if a company doesn’t trust you it doesn’t really matter what the motivation is you might as well use a self signed certificate.
Paid certificates have the money to make sure everyone trusts them and has a reputation to maintain so are more likely to defend a legitimate complaint.
99.999% of individuals it simply doesn’t matter(although you might have to look into it if you’re using android apps) but to a company the little bit that certification costs is worth every penny.