Because, in most cases, their rolls are a result of a choice. If everything went well, they have some information and decide to act upon it. You translate it and say "give me a something something roll" or "roll to hit". They willingly took a risk, and it succeeded. It's a gamble, but one they willingly take.
And if it was a one-and-done roll, then sure I could maybe get behind what you are saying.
But a combat involves dozens upon dozens of different rolls... attack rolls, damage rolls, saving throws, ability checks in the moment etc. To think every single one is "important" due to the "choice" a player took to get to that roll is a little extreme in my opinion. If a player has stood in front of an enemy and is making their seventh attack roll against that enemy because they are just in a mano-a-mano fight... what real "choice" did they make that was so important? They aren't that hurt and their enemy isn't down yet after a bunch of hits-- so it's either keep doing what they've been doing, or else run away for what would seem to be no real reason. At the end of the day, there really isn't a "choice", because there's only one rather logical action to take... the same exact action they had taken six times previously. So that "choice" isn't really so special that I need to hold it up like a Faberge egg.
But be that as it may... for me and my players it comes down to the functionality of combat. What is getting into combat supposed to accomplish? For other tables, it might be the tactical board game-- the pieces are on the board, everyone has their stats, and now it comes down to out-strategizing the DM in order to win the fight. So the same way you wouldn't want to cheat at Risk (because otherwise, why play it?)... you wouldn't want to cheat in the D&D board game. And I get that, absolutely 100%. For a large percentage of players, the board game is sacrosanct. It's why optimization is such an important thing for a lot of folks-- they want to play the strategy combat game to the top of their intelligence (and which also explains why so many players hate when WotC removes restrictions from character creation, because then if they want to play less stereotypical and optimized PCs, they have to purposefully nerf themselves on their own, rather than relying on WotC to do it. Which makes them feel like they aren't playing to the top of their intelligence.)
But for a lot of the rest of us... combat is just part of the narrative. The fights add drama to the story of these characters. But the combat are no more important than any other part of the drama. I mean for me... if I'm in a roleplay scenario with the PCs and their characters make really good arguments in their communication with whomever I'm playing as an NPC... I might forgo asking for a Persuasion check altogether. They won the argument, so I'm not even going to make the roll. Essentially I'm "fudging" the roleplay scenario by automatically letting the PCs win their "fight". That kind of stuff happens all the time. If the players explain a really awesome method for getting up the 50' waterfall, I might "fudge" and say they don't have to roll Athletics checks even if I had written down it was originally a DC 15 check to do so. And I don't treat combat really any differently. It might involve MORE dice rolls than climbing a waterfall... but I don't see them inherently more important. And thus "fudging" is not the end of the world for my table.
And as far as "lying" to my players... guilty as charged! As was said above... DMing is like being a magician-- turning talking, numbers, and dice into a fun and awesome 3 hours of drama and excitement. And I will lie my ass off to make sure my magic tricks work. And my player do not care in the slightest nor ever ask how I'm doing it, because they know coming into the game that D&D is entirely a magic trick, and they are looking to be fooled. Because being fooled is part of the fun. And obviously, other tables will certainly disagree and feel differently. But thankfully I don't play with any of you weirdos.

LOL!