Why is half-life 2 so great? (no trolling please) (spoilers if you can even spoil a 10 yr old game)

Ok, so i finally went and did it, i played through half life 2. Now I got to nova prospekt and was willing to give up because the first half of the game is simply boring. you are airboating through the abyss with the occasional helicopter. You are traveling to eli's lab and then black mesa east without really any reason other than alyx told you to. My friends told me to keep going so I did and yes, the second half of the game is a pretty intense shooter. So it definitely redeemed itself. The r-town is boring now a days but i see how it was creepy back in the day.

So you can say the second half of the game is great. No problems there and if you wanna argue the first half sets the scene of one man against the world i will give you that. But here is the problem i want someone to explain to me. The story is almost non existent. You go along with alyx saving her dad and overthrowing dr. breen, but as a player, I had no motivation for such acts. What is city 17? How the hell did the Xen end up all over the city? What makes the gov't so evil? What is all this talk of immortality? I read all about the answers to these questions online but that doesn't count, it was not explained in the game, and i haven't played the episodes but the game was praised on its release and the stand alone title is the one that shows up on best of lists.

2 things I want to state. 1) this game was released a week after halo 2. I feel like in the collective memory of gamers it is older than that. Yes it has dope graphics and physics but compared to halo 2 and call of duty 1 the whole shooting/walking/drive without any visible arms/ pick stuff up by floating it in your face/walk up ladders underwater like its 1995.... dates the game very very badly and also, compared to the ridiculously fleshed out story of halo 2, i don't see how the story and mechanics weren't gripes at the time.

2) valves similar effort portal 2 (portal one doesn't count cause it was pretty much a side project) was all about story. it was similar to half life with no cutscenes/a silent character but you learn all about the history of aperture science and the origin of glados. PORTAL EXPLAINS ITS WORLD HALF LIFE 2 DOES NOT. <----take away

I am not trying to hate, i just cannot wrap my head around why it is exalted as the greatest fps of all time. and i would love someone to explain how the lack of story isn't a problem, or where i missed the explanation of the story.

also, for the record, i played most of half life 1 and for the time and the fact it has an actual story i would rank it higher