There is ONE monster of CR4 with 120 HP, weretiger, but paired with horrible 12 AC, rest are from 97 HP to 36 HP, averages around 65 HP
Take up the issue with WotC. I'm using their own monster CR math, directly published in the book. (You can see
a copy of these numbers here, where the author refers to them as "pretty much right" amongst other things.) If it's wrong, they're literally telling people to create monsters with way too much health. Not that this surprises me in the least. It's an open secret that numerous MM creatures don't actually follow the claimed math for determining a creature's CR, and I have seen credible claims from credited consultants that in most cases this is because their CRs were tweaked ad-hoc due to playtesting.
Also, that's only counting monsters in the MM. There are several more CR4 monsters than that--e.g. at least one in MPMM, and several spread across various adventures, which have roughly that amount of HP (e.g. the Clockwork Stone Defender from MPMM has 105 average HP and is a CR 4 creature.)
so yeah, if you take tankiest CR4, you will need on average 5 attacks with +3 greatsword and GWM, but for most you will need 3 attacks.
and if you are facing several of them, it's time for action surge, 8 attacks with 15% crit chance and almost certain 1 killed per round will give you 9th attack as bonus action.
2nd round again action surge, that is 18 attacks in two rounds. That certainly looks impressive.
Then bump it up to CR 5. From what I can see, five different creatures solely from the Monster Manual have at least 120 HP at CR 5, and Volo's, MPMM, Bigby, and numerous adventures have added more. The problem remains effectively unchanged.
And fine, sure, let's look at the actual stuff here. Remember, the vast majority of the damage being dealt here comes from flat damage (STR, magic weapon, GWM)--critting, even on a greatsword, only adds ~7 damage, or ~8.33 with GWF. Not that impressive compared to the 18 flat damage (5 STR + 3 weapon + 10 GWM).
If using GWM, that's a -5 to hit. Average AC for a CR 6 creature is 15, so (with a +3 weapon) that's a net 75% hit chance (of which 15 points are crit): .6(8.33+18)+.15(2×8.33+18) = almost exactly 21 average damage per attempted attack. Given we're making 8, possibly 9 attacks in a given round (blowing through all the Fighter's resources in the process), we can probably assume that that average will be reasonably accurate overall. That means ~168 damage, give or take, per round. That definitely does (over)kill one target. So you can kill...at best...two of these things per round. By blowing
everything you have to do so.
On any other round, you're doing ~84 damage. This is not enough to kill a single CR 5 creature in one turn, even on the basis of the average HP of actually printed CR 5 creatures (rather than the instructions for how to build a CR 5 creature, which claim such creatures should have 131-145 HP, not the
90 HP that is actually average for CR 5 creatures). A party of five level 20 characters going up against eleven CR 5s is
not going to be having a "wow, look how effortlessly we're stomping these" even though they're literally
fifteen levels beyond fighting such creatures as a main focus. The claimed experience--that things you once saw as deadly threats, but now see as gnats to be swatted aside--simply doesn't exist with 5e monsters past CR 3 or 4. It
emphatically doesn't exist for monsters of CR 7+.