Because we are having 2 races at the same track back to back, they're just doing something to spice it up a little, that's all.
If they all had the same setups and same tyres it would be a bit too easy.
Mercedes are the class of the field this year unfortunately, last year Ferrari kept them honest and won a few races themselves, it was much more competitive.
Certain tracks are better for "all action" races. Unfortunately some tracks are designed with older cars still in mind, and the modern day f1 cars are just too fast and can't get close enough around corners to overtake properly. They hope that with the rule changes due in 2022 overtaking will be easier.
Monaco is literally the most boring track for "action" as there is zero room to overtake anywhere. The excitement comes more from the cars being centimetres away from the barriers and at risk of crashing almost every corner. But for the average viewer Monaco is really boring tbh.
BTW, for people wondering why they had tyre issues this weekend - it was purely down to the safey car coming out twice.
The 12 or so laps spent behind the safety car effectively removed the need for an exta pitstop. Usually Silverstone would always have 2 stops, but the teams thought they could stretch it out by driving sensibly. Obviously that was a bit closer than they expected.
Next race could end up having 2 or 3 stops which would make things livelier.