The simple answer is because they're football clubs, not kit manufacturers. (independent.co.uk)
Antonio Rudiger and Tiemoue Bakayoko have both liked a post from Chelsea's new kit manufacturers Nike on Instagram. (fastonetwo.com)
There is ZERO reason why UTD should earn around # 21 million more than us from kit manufacturer when they was in the EL and we was in the UCL, we had 2 of the top 5 kit sales in the EPL with Ozil and Alexis! (justarsenal.com)