Because the name is a contraction of T. E. Stockwell and Jack Cohen.
There is a tendency to make store names possessive even if they aren't.
I usually hear about this in the US, I can't recall a lot of UK examples, but I think we have it here.
I had a chat with ChatGPT earlier who said much the same thing, but I was unconvinced by any of the examples other than Marks and Spencer(s). I think Tescos just feels nicer to say?