1. Because they are so margin driven. This is not the first time MB has cut cost for the sake of margin. They are well aware of Tesla's current market dominance and the Chinese are literally flooding markets with low priced EV's. MB's current EQ range has no demonstratable drivetrain advantage so the product price must be as sharp as possible. If Tesla can get away with shit plastics, they are probably thinking why can't they.
2. They wrote the book on how to screw a car together, but they have stayed from their own formula on many occasions.
3. The door panels on all of the EQ's are hard plastic, the door handles literally flex when under pressure, the dashboard trims groan and creak (akin to the w205 C and the W213 E), the door bins are not even lined, the panel gaps are clearly not within normal tolerance thresholds and I can't believe dealerships aren't even adjusting these before the cars are put on the showroom floor. I have previously posted videos demonstrating some of these issues.
I have driven an EQS53, EQE53 and have been chauffeured to the airport in the EQE450. The ride and refinement of the EQE450 was good, but the quality of the cabin was no better than a C-class. The EQS53 is not even close to an S-class in overall ambience and finish and build.
I have recently as last week attended an MB event where the some senior reps from MB OZ who visit MB Germany for product previews etc. were quite candid with me in acknowledging the build and finish issues of EQ's. In one's own words, 'The EQS is by no means an S-class, and should not have been marketed as such.'
Take the above for what you think it's worth, but there was clearly a deliberate decision to rip out costs from the EQ products where they could. What I don't think Mercedes acknowledges is a lot of their buyers would pay some extra $$ for upgraded finishes.