Because my GitLab self-hosted instance is up currently and theirs isn't.
Without numbers, I don't see how you can make any decent conclusion at all.
Is that all you have seen? I have seen many self hosted services fail in our company and the answer is almost rarely any of these simple things but complex things like data loss, data corruption, random restarts, network partition, configuration sync issues etc. That is why companies pay other companies for critical things like gitlab even when self hosting.