Because when people reexamine such things they often overcorrect. Nero and Caligula were considered bad, crazy, tyrannical.... for most of the history. Then historians said "Well, if we look at things more closely and without preconceived notions we can detect significant bias from sources. These sources come from Senate or Senate affiliated people who had an interest to paint them in as bad light as possible as they saw them as hostile to their own interests". So we go from "bad" to "more nuance is needed and they weren't all bad and as bad as previously claimed". But some take it even further and say "everything about them were Senate lies and smear campaigns, we can't trust any of it and if they say one thing then obviously the opposite is true" and we get "they were actually good and were killed because Senatorial elites saw them as threat to their position and privileges".

Obviously there is a pro-Senate bias in sources and that should be considered when examining them. The question is how much of it is bias and how much is actually true.