Because the cost of the waste these throw-away phones generate isn't priced in. There is a societal cost that isn't being paid by the firm or the consumer.

Honestly, it probably only adds a few bucks to the BOM and the consumer who doesn't care about repairability still likely comes out better off in the long run because their phone lasts them longer and/or resells for more because it isn't junk after 3 years.