This. I also love how the game takes the linearity out of the Action genre. I would probably list it as semi action-adventure because of that. The integration of the puzzles is great as well. It isn't obvious what you can use for a puzzle, and it all is real world.

becasue tis good?

There aren't very many single player FPS games that can compare to HL

I did a lot of mathematical calculations and I was very impressed by the result.
Here's the formula:

Half-Life 2 = best FPS ever

Let me set the FPS scene when Half-Life 1 came out:

-Most FPS games that came out were in the vein of Quake or Unreal: unrealistic arcade shooters with very little narative.
-Quake 2 was one of the top FPS of the time, but in SP the A.I was so bad the enemies would try to get at you by running into walls
-FPS' were brown... with bits of grey (some things don't change much do they?!)
-In most games it didn't matter where you shot someone, the damage would still be the same

Then came Half-Life and we got:

-A pseudo-realistic experience where you had to reload your weapon with realistic environments
-Amazingly intelligent (for the time) A.I. Enemies would try to flush you out with grenades, they would take cover, they would run from grenades you threw at them saying "OH SHIT!" Might seem silly now but this was all revolutionary at the time.
-We had some nice colourful environments
-Shooting someone in the head actually killed them! More powerful enemies could be taken out quickly with just a few headshots rather than having to shoot anywhere and it doing exactly the same damage.
-An actual storyline

Basically, without Half-Life 1 we wouldn't have had the vast majority of modern FPS such as CoD, Halo, Crysis, Far Cry, Left 4 dead, Counterstrike, FEAR etc.

Half-Life 2 added even more story telling elements, a wide range of different tasks from vehicles to zombies to puzzle solving to commanding small teams etc. Then it also incorporated the Havok physics engine far better than any game before it, and in many ways better than many games out now. The gravity gun was also revolutionary and now a lot of games incorporate a similar gameplay device that functions in a similar way.

Half Life 2 + EP1/2 is one of the best FPS experiences ever. Unlike most games that start you with an assault rifle, you spend a good amount at the start of HL2 with only a pistol where you can't just take on every enemy you encounter. There are a few points in the game where you only have crappy weapons, or there's no ammo so you need to use your gravity gun (ravenholm). The starting gameplay with only a pistol, or when you're in the dunes shooting down an attack chopper, or defending an outpost from an onslaught from massive walkers are all massively different. The atmosphere in this game is second to none.

Unlike most modern games, there are no sections in HL2 that feel phoned in. Many modern FPSes have the obligatory drawn-out vehicle section, some other gimmicky gameplay element, or are generally lacking as good single player experiences.

On the 7th day God rested and created Half-Life 2.

And it was good.

Let me explain my experience with the Half-Life series, and maybe this helps:

I played Half-Life after it first came out.  I recall the leaked OEM demo of Day 1, and how that generated a buzz.  A demo spreading was said to be "illegal" but people played it anyhow.  Well, I then got the game and played it.  It brought to mind old adventure games, and a world that felt right and logical and real.  The scene early on with the opening the gate to the other universe was creepy.  It just worked for me, and was a fine point.  I thought it was real good, but didn't think multi-player would amount to much.  How wrong I could be.  The game became a staple of the industry, giving birth to Counter Strike.  The backstory was solid also.

Now, go to Half-Life 2.  The art of game making is extended further.  Well, at least I thought it was.  It was real good, and worked, and extended further, and the game was mod friendly.  It extended the reputation of Half-Life 2.  Maybe now it is dated.  But consider what it was when it first came out.  The game added to the industry.

And then, there is The Orange Box, which is one of the best values ever.  And it comes with both Team Fortress 2 and also Portal.  Both of which have been praised, and Portal being considered one of the best games of this generation.

Half-Life is respected because it delivered on the expectations people had for it. 

It feels so realistic almost like a real world.

Because it is that good?