u/Temponautics addressed why why medieval lords couldn't promote themselves via self-declaration in the thread Titles like "Emperor" historically held a lot of prestige, and not very many people ever held them. Was there something preventing rulers from just declaring themselves Emperor that led to this relative rarity?

I touched upon general principles about why one needs to be really careful on how did people self-declare Emperor.

In short, a title holds meaning based on history, political reality and tradition. Seeking to change it risked upsetting both powers abroad and figures at home while risking undermining one's own legitimacy to rule. Titles changes have to be handled with care to ensure legitimacy and survival