You're right the players should just show up end of July and play a few games before the PL starts
I mean what on earth are the club spending a month of fitness work before playing games right?
A proper pre-season puts players to the limit in terms of fitness and conditions them optimally for the season and even then once the pre-season matches start they typically play 40-50 mins and build up their fitness
A player cannot simply do the level of fitness work that he needs to do in pre-season during the middle of the regular season nor consistent intensity of starting 90 mins week in week out without conditioning will lead to physical fatigue or injury (Glad we didn't get the latter at least with Havertz out)
it's really not rocket science
There's a reason all 3 Isak, Gyokeres and wissa all had issues