I'd see this as a red flag and look for a different photographer. Some time ago I've had similar issues with a designer I hired: He wouldn't give me vector file

I'd see this as a red flag and look for a different photographer. Some time ago I've had similar issues with a designer I hired: He wouldn't give me vector files, only flat PNGs. He said it had to do something with his reputation. Obviously, these people want you to come back rather than hire someone else if you need any changes, or additional prints later on. That's why they only want to give you low res files. But for the customer, that's a bit troubling. What if it turns out the photographer makes lousy prints? What if the photographer closes up shop? It doesn't matter if this is "standard business practice". This attitude is typical for people who don't take their customers' demands seriously. Luckily it turns out there are plenty of professionals who care more about their customers than about their reputation. In my case, I found a different designer, who understood my point of view and worked with me, rather than say "can't be done". Unfortunately, he was way more expensive, but in the end it was worth it.