Because Nike pay us an obscene amount of money to sell shirts with our name on.
They're not gonna give us tens of millions a year and not be able to recoup that money.
They also make kits for Brighton, but they cost £52 for a replica shirt Vs £70 and they don't make an Elite one. Nike obviously don't pay Brighton anywhere near as much as us, Chelsea or Liverpool, ergo, they sell their shirts at a lower price to recoup their "investment"
So, if you want cheaper replica kits, you need our kit maker to sponsor us less money. Then you can have a cheaper shirt and we then can't offer Son and Kane the money they are entitled to earn (vs losing them to a rival who will pay it)
Hope this explains it.