Halo 1 is better on all 3 aspects, though.
To each their own, I marathoned a bunch of FPS games; Prey, Halo CE, Halo 2, Metro Exodus, Legend: The Box, COD: Infinite Warfare, FEAR, Half-Life, Half-Life 2, Titanfall 2, and Halo 3 all in a row during the pandemic. Besides Legend: The Box, the Halo games were by far the least enjoyable single player campaign experiences in that run, and always felt like a slog to me.
While Halo CE hasn't aged particularly well, I think it does have enjoyable mechanics that I would admit are a little more accessible than jumping into an old source engine game. That said, I don't think any Halo's single player campaign has come close to matching Half-Life 2's presentation, pacing, art; I feel like a more apt comparison is Halo CE to HL1.
Halo's mechanics certainly do hold up better than the jank platforming sections of HL1. It doesn't have the weird ramp up in difficulty like HL1's surface tension level, but overall the pacing and variety is way, way worse. Halo's copy-paste level design within the corridor sections is horrid.
Halo's open battlefields aren't bad, but are also kind of jank. I've had a lot of people say Halo CE is the only game that feels like you are authentically immersed in a huge pitch battle. But you can oftentimes just bypass the action and walk broadly around the AI vs AI. These types of AI vs AI battles were done in 1998 by Half-Life as well. AI Marines aren't complete idiots, but games like Half-Life also had assist characters. They ultimately do little to help in either game, but in Half-Life they do a better job of drawing fire.
While Halo is lauded for it's enemy AI, which is not bad, sometimes it felt like the enemies were just moving for the sake of moving. Not taking on any sort of strategic attack. The enemy types are limited, so I feel like they ultimately become predictable and the game just feeds you the same clusters over and over again in repetitive game play loops, whereas Half-Life often finds novel uses for it's enemies through level design variety.
In the end, you know Elites are going to be tough, Jackals are going to be defensive, and grunts are just going to run away. The flood is on par with Serious Sam 1 villains; no AI to speak of, they just bum rush you and are an absolute slog to fight from the get go. Whether or not they constitute as much game as Convenant enemies or not, the flood feel like they take up 3/4ths of the game for how mindless and obnoxiously repetitive they are. The flood sections of all the games are like pulling teeth for me.
Anyway that's my two cents. Way, way more egregious than OP's assumption people don't like Half-Life 2, I am sure.
I know Halo is hugely important for the videogame world, and I'll absolutely buy Halo Infinite on day one to support the release and share the experience...but honestly, the legacy games were always kind of a chore to finish for me and certainly broke the pacing of my recent FPS marathon.