Why?
I believe the blood, time and treasure the WAllies would have to pay with no Red Army to kill millions of German soldiers and no Eastern Front to soak up massive German resources would give them pause even if they continued fighting.
The US/UK leadership were still human beings that cared about the loss of human life unlike Stalin and the Politburo.
Because leaving Hitler and co around is just going to lead to a lot more blood and loss of life, it already had
You also seem to forget Britain lost 2-3x as many fighting in the last war, British and American air raids killed what a million plus German civilians, in WW1 we were about to starve Germany to death
This idea the the walies are scared of death and killing when it comes to stopping Hitler or just in general 'not doing what needs to be done' just because we don't post Russian or German loss numbers is really one of the most pernicious themes of these kind of threads, especially when it's seemingly only ever used as an edgy excuse to let the Germans win for a more exciting ATL.
The Wallies didn't post Germen or Russian losses because we were less committed to the fight it's because we had better or just different options for fighting, and we worked hard to get them and worked hard to leverage them.
Also Stalin didn't send a 20m Russian to die because he relished sending people to die*, it was fight and die or just die, because Germany victorious is going depopulate Russia! (also frankly this also buys into the "Russia just zerg rushed their way to Berlin", which is a much over stated meme, yes they had a numbers advantage but it wasn't the only reason why they beat the Germans even if this gets in the way of the "German quality drowned in Soviet masses" narrative)
Plus as pointed out no the wallies are not going to land millions of troops and meat grind their way across Western Europe just so that this inherent weakness you think you see will be exposed. They're going to bomb the Reich and then invade and fight far weaker Reich if they need to.
Because time and treasure was a luxury the Wallies had that the Russian and the Germans didn't
*don't get me wrong that doesn't mean he wasn't a ruthless dictator with the deaths of millions on his hands due to his policies