While the ease of not needing to make an argument might make it tempting to simply pass it off with gm style, it's a problem that too many good experienced gms with no trouble creating challenge in too many other systems have that problem with 5e& I've seen it too many times at too many tables to simply pass it off as a gm problem.
Or Think there are a couple better possibilities. In no particular order
- Your players are just bad at whatever they are playing (what's the group makeup & level?).
- Your players are too low level. The system begins breaking down around 5-6 as too many abilities start coming online that work together to make challenges irrelevant up until the point where the system almost immediately switches over to the players just never stood a chance somewhere around 8-12 they've finished & the game crumbles to what might as well be goku the supersaian & kal'el the kryptonian vrs school children.
- Your players are very under equipped. A level 8 party should have at least a few magic items, xge gives some (bad) guidance on it somewhere
- The party makeup is just bad (what is the party makeup & rough levels?)
- Your players are actively fighting the urge to look at the blinding light streaming out from the cracks in the system or you yourself have not noticed them peeking behind the curtain to realize the wizard of oz (ie difficulty) is a farce
You also mentioned
"The amount of fun we have had nothing to do with the number of encounters or resources spent. " in another post, & that's true but missing a key part of why the inability to challenge players without resorting to apocalypse engines & other absurdities is a problem. Namely, doing so destroys worldbuilding & anything the party had shepherded into development or forces the gm to end & restart the campaign with new characters just as the party is excited about having their abilities
(not freaking capstones8-12 levels away) come online.