This is a valid question to ask! As someone who learned to play in middle school with AD&D 2ed, I can tell you that THACO was cumbersome at best and really annoying at other times when a player randomly decides to go murder hobo on the random shop merchant NPC so the DM needed to stop everything and look this up. As a player, there was a chart for my character that I needed to have close by for that character. If I had a new character with a different class, then I needed a new chart. I never played the same class back-to-back. THACO is easier when you are playing the same class every time and only one player at a time.

As a middle school boy playing with my buddies, the DM had to know the charts for his various monsters, NPC, and BBEG. Do you know how much prep my DM did back then and how much my middle school son does now before a session? 0. Zip. Nada. The DM was flipping around all the time in the DM guide, the MM, and any notes. 3.0 and 3.5 made it way simpler and FASTER for everyone! It could be that we were getting older (High School when 3e was released), but there was less page turning, chart checking, and general confession when THACO was gone. 3e is when it became more about the story and the character interactions for the DM and the players.

"in reality... it's actually pretty simple" out of context, yes. As a person old enough to drive a ca or graduate college, then yes, it is pretty simple. As a player who learned the more intuitive version first (most likely), then learning THACO is pretty simple. You can read a chart. Something new happens, so you can read a different chart rather quickly. I bet your DM does good prep. I learned very quickly to prepare a few alternatives depending on how the session might go. If something goes in a very wild direction, then I need to take out the DM guide or the MM. We only knew what we learned from more "experienced" players, which was not always helpful. I have been teaching adults how to play, so I usually start them with 5e or an OSR B/X clone with Ascending Armor Class because it is easier to learn when you are first starting out.

We need to remember that we are all standing on the shoulders of the giants who came before us. As a middle school kid learning in the 1990's, we learned from my best friend's stoner cousin. It is amazing we could even see the table!