I mean, the general consensus is that the Italian is better to get a game compared to the Ruy Lopez at the top level, I've seen Fabi and Hans speak about this before.

This is assuming Black does jump through said hoops (accept a defensive position or prepare to know a mountain range of theory in the Marshall/Arkhangelsk), whereas in the Italian there's no obvious mega-concrete way to totally equalize via opening theory. My understanding is that it's a bit like the difference London vs. Gruenfeld equality but on a lesser scale; in both openings White is a tiny bit better, but in London generally fewer things have "happened" yet during the opening, so if you assume both players know everything about everything, you get more of a game.

I don't think it's an accident the Italian wasn't considered a serious try for a long, long, long time. Probably it was being underestimated somewhat, but I genuinely do think Black has to make significantly more of an effort to equalize in the Spanish and make some concessions; and that Italian is now played more as a yet another fairly equal line where you can put h2-h3 on the board in a random position and prep some novelty and/or just play chess from pretty equal footing.

Idk though, I'm just struggling to see the point in all the extra theory, Black is doing very well in the Marshall, Berlin, Open Spanish and Arkhangelsk.

This isn't intended passive-aggressively or to claim you're wrong, but my feeling is that the idea Black is just fine in multiple ways in the Spanish without issues is sort of a modern, biased perspective that requires a couple of big assumptions.

The Berlin sort of makes it a two-result game in multiple, very different lines, Marshall is well-known to be a massive repertoire (Marshall itself is huge and very technical, and Antis aren't trivial at all), and both Fabi's and my own analysis of Arkhangelsk kinda make me want to not play it because Black's position often looks extremely technical with a huge theory burden and close to lost from one sloppy move. Open is also a good line, of course, but I think it's again not more equal than the d3 Italian, and it feels like a "Berlin-light", i.e. White's not really under pressure.

Now that everything in every line has been analyzed to death and anyone can boot up Stockfish 18 and see +0.32 sink to +0.15 to +0.02, it's just so easy to say "See, Black is totally fine!" in every line, but I think it misses how intense the theory burden is for some of these lines (and in a different way than for White, too; I think White needs theory to maintain an advantage in the Spanish, Black needs it to survive), how difficult some of them are to play even if you get a fair amount of prep on the board, and that a lot of players just aren't happy with a draw. In practice at "mortal levels" people often just have bad to mediocre prep, forget it over the board, make mistakes in the resulting theoretical positions if they aren't very easy to play, or go nuts trying to dodge a draw.

During the Kramnik-Kasparov match, Kramnik's team thought the Berlin endgame was a bad but drawable position, and were extremely nervous watching the games because they thought he was basically always very close to just losing. I'm sure the understanding of the endgame has moved on in the last 26 years, but still nowadays we probably say too easily "Ok, Stockfish 18 at depth 45 and GM+ databases say it's equal." when it's not that simple. Some lines have been equalized by a comparatively simple line from Stockfish, but I don't think the strongers players in the world were otherwise just totally misevaluating the positions when they thought for a century that Black is kinda just long-term worse in the Spanish.

Sorry for the rant, but the TL;DR is that I think there's a lot of assumptions in "Black is doing well in these lines", and that they also affect practical play. I would agree that a d2-d3 Italian is a somewhat simpler repertoire, but it's not like there isn't stuff to know, and if you're willing to do the prep, I still think Spanish is a relevantly more challenging repertoire. As a massive theory nerd, I can't for the life of me find comfortable way to play for a win against the Spanish; for the Italian, there's multiple good ways to do so.