This is why you aren't a business person. Microsoft KNEW if they didn't release it on Steam, they were SEVERELY limiting the amount of people who would buy this game. The Microsoft Store is hot garbage on PC, and I hate that I had to use it for Gears 4 and Forza Horizon 4. A huge number of PC players actually refuse to buy anything on the Microsoft Store because of the way it handles game installations. I am one of the few who did install games through it simply because I wanted to play the games. Putting the game on Steam, the biggest store for PC games, gets it in front of more consumers, thus increasing the amount of copies sold.
Also, The Last of Us being multiplatform is not the same. Yes, you may consider Steam a "competitor" (I don't; they're just a store. It's like a digital Best Buy) but guess what Operating System the computer has to have to run Halo? Windows.
Also, game sales ARE the most important way for these companies to make a profit. Yes, consoles do eventually make a profit after the component prices come down in price the older they get. Also, Harty is right. Nintendo games stay full price because you can't get them anywhere else. Target even has to exclude them from the buy 2 get 1 free deals they run sometimes, because Nintendo tells them they can't discount them (this may have changed, it has been a while since I participated in one of those sales.)
Microsoft is not getting rid of Xbox Consoles. They know there are people, like yourself and even me to an extent, that want to lay back and not have to worry about if a game will run on their machine, and that's one reason Xbox consoles will stick around. They are just expanding their reach to people WHO DON'T WANT ONE (or someone like me, who has one, but would prefer to play games on PC if they're available there.) Say it's the year 2018, someone wants to play Halo, but they don't want an Xbox. What is the likeliest thing to happen? That person just doesn't play Halo. Microsoft doesn't make money from that person. Now, say the same person does have a PC capable of playing games, and Halo is available on PC. Microsoft makes money on them by providing them a PC version, which puts more money in Microsoft's pocket. They GAIN a sale they otherwise wouldn't have had.
Also, just judging from the reaction from the Halo and gaming communities, you are in the minority. The vast majority of people are very excited that Halo is finally coming back to PC. More people coming back to Halo is a great thing. This also gives PC players the ability to FINALLY finish the original trilogy since Halo 1 and 2 were already released on PC.