The reason is simple - in order to win the Premier League, you will need about 95pts, and that means you can only afford to drop about 19 total.
Factor in games against the top 6 and there's basically absolutely no room now for dropping points against midtable teams or lower.
In most leagues, the bottom half teams roll-over and/or go blow for blow against the top teams, invariably getting beaten in the process.
In the Premier League, clubs like Wolves or Burnley will turn up at Old Trafford and literally setup and fight as if their entire careers boil down to scraping a 0-0 against United. Look at the pattern of results teams get after drawing with/beating United. They normally exert so much effort into playing us that they get beaten the following week. Check out last seasons results.
All of this means that we need to find a way to create enough chances to beat these crab-like defensive teams week-in, week-out...and because there's no space in behind and they pack the middle of the pitch, football tactics have evolved to create attacking overloads in the half-spaces and also to win the ball back high off opponents, relentlessly press them back into their own half, isolate their forward(s) and score from transitional moments.
The problem we have is that we didn't play this way for a decade. LvG perhaps came closest but his style was dull and the players didn't seem to buy into it. Jose was a bit of a throwback. OGS seemed to rely on counter attacks, but predictably really struggled against teams who wouldn't let us counter them. Remember how good our away record was, but also how many draws we got at OT against relegation fodder?
People are so quick to whinge nowadays that they aren't looking at the underlying numbers. Last season we had the most shots and created the most big chances but had an absolutely shocking conversion rate. This season, we've already had 50 shots, the second most in the league. Again, our conversion has been shocking. This all matters though, because its a work in progress and building a squad cut out to play this way takes time. Rashford isn't, he's lazy, can only play one position and doesn't press well enough. Martial very similar, bar the 'one position' criticism. Sancho similar, although think it's more lack of physical attributes and timidness rather than laziness.
ETH also needs a CF he can hit with a long ball. Haller at Ajax did the job for him and he loaned Weghorst for that purpose (he was garbage at it, but still better than Rashford).
You can't have your cake and eat it. One United fan famously demanded 'a barca style of play'. Well you don't get that overnight and you need a squad with a very specific skillset to pull it off. Fans don't see it yet but Anthony and Mount have the skills that Rashford, Martial and Sancho lack.
I'm convinced ETH will pull it off but it will take time. If people want to revert to the double pivot and the middle block and finish 4th year-in, year-out, then that's their perogative. I believe ETH has higher ambitions than that and is set about building a team that can deliver major trophies.
What we're also going to see is a continued move away from 'flair' players towards functional players. That doesn't mean not talented, but good at different things than we are used to seeing at OT.