Mouser is a warehouse supplier. It can only sell you stuff that they have...
Stuff that is not in stock is still available with lead times so I'm not sure what you are saying there
... and you actually dont have a contract with them about the delivery.
They took my money and then sent me no less than 2 emails over the space of a week stipulating the delivery date, I don't think it gets more contractual than that, although I'm sure the terms and conditions will make swiss cheese look air tight.
Those lead times are not reliable from Mouser. If you order from them, maybe you get your parts, because someone else ordered a larger batch of parts. Maybe because Mouser expected to need those parts. As I said, you dont have a contract.
If you order from Arrow and the others, the manufacurer will start the production line from you. Or they tell you that this is when the production line starts next time, and you will get your parts from them. The difference is that with them, you probably sit down at a meeting table (or online in 2020), ask for a quote (which is legally binding BTW), they tell you the conditions, you can negotiate price and payment terms, and then you have a contract.
Ignore the website of all these companies, they operate at a completely different manner.
Not only it is a lot cheaper to buy from them, you get a totally different service.