What makes frozen burger patties bland? This has confused me for a while: it's observable that a lot of all-beef burger patties from grocery stores and fast food joints taste dull compared to homemade ground beef, but is the key difference in the fact that they're frozen, or is it in the cuts of meat that they use?

For instance, I had a sirloin burger from McDonalds that tasted only marginally better than their norm - assuming that they're not lying about the cut of meat, does that mean the patty was bland just because it was frozen?

If that's true, then is it a bad idea to grind your own meat and freeze it? I'm asking because I wouldn't mind just grinding a bunch of chuck instead of buying preground meat if it'll taste as good as the homemade burgers I've had, but I wonder if there's a catch.

And yet it'd be strange if freezing ruined the flavor, because I assume grocery store meat is previously frozen anyway. So I'm not sure what to make of this.
When you buy a burger from a fast food restaurant you are buying a product that has been engineered to provide absolute consistency, be very fast to cook, and be as cheap as possible so it can be offered at a low price for a reasonable profit. The grill marks are engineered in, and they are given coatings which will give the right appearance when cooked. They also have flavorings added so they taste as the designers (yes there are burger designers) intend. It's not a simple bunch of ground beef at all. Most frozen pre-made burger patties are similar, they use cheap cuts of beef (and sometimes reclaimed meat) and artificial ingredients to produce a consistent and inexpensive product. They also tend to be very thin and are easy to cook all the goodness out of by accident. 

If you want a really good burger then you need good burger meat, it's as simple as that. You can't make a good burger out of low quality meat no matter what you put into it. The simplest way to make a good burger is to find a local source of good quality ground beef and make your own patties. Making patties is very easy and quick, just get your hands in there. Many supermarkets have perfectly good packaged products (I get mine from a local grocery store and it makes good burgers), so that would be my first port of call, next would be butcher shops. 

If all else fails and you want to make your own (or just give it a try) then you're in luck because the best cuts for burgers are also the less expensive ones, like brisket, chuck, short rib, and round. Round has great flavor but is very lean, and working cuts like brisket and chuck are also lean, so you'll want to add some fat. Many burger nuts say you need 30% fat by weight, personally I'd aim for 20% as 30% is too much for my taste, but that's up to you. 

As for frozen versus fresh, freezing does cause some loss of flavor, however it's not much. You are much better off with frozen good quality meat than fresh low quality.