My friends think slow and it irritates me.

Unlike in the old west, the slow draw wins…

But seriously, it’s better to say something at the moment rather than to remain quiet and grumble about it later.

Maybe your friends are/were on a tight budget and were embarrassed to say so. I’m thinking some kind of drastic reduction in their income brought on by the economic crisis; possible some bad news received after the plans had been made. The nature of your plans (supermarket, park) suggests that some or all of them were on a tight budget.

I think some posters are misinterpreting the OP’s comment regarding the advice given by his/her parents. If I’m not mistaken, what he/she is saying that if he/she had been in the position of his/her friends, he/she wouldn’t have hesitated to offer payment each time.

In any case, now you know better, right?

Expecting friends to behave decently and pay their share of expenses is not a character flaw or a sign of stupidity. The friends were rude and thoughtless to not offer to chip in.

That may be, but it’s not an example of his friends thinking slow.

I suspect the OP isn’t going to come back, but I’d be interested to hear what he/she said in reply to this friend.

That is a hypothesis as to why otherwise-decent people would behave this way. Surely the OP would have noticed by now if these friends are the type to do this often.