so what's wrong with buying seed from a big box store?

"so what's wrong with buying seed from a big box store?"

They don't have seeds of elite cultivars and the seed they do have tends to have a fair amount of weed seeds. The percentage looks small, but when you calculate how many weeds it really means, it adds up.

I agree there are good seeds and there are bad seeds...one has to trust from where the product comes from.
The buying of seed is no different than buying a sofa or ...heavens...a car; one must trust and maybe know something what we are looking for.

In your zone 5, it has been reliably shown that a mix of Kentucky Blue/perennial ryegrass and fescue makes for a decent lawn that will standup to normal weather that is prevalent in the area.
It is though, a good idea to always buy the best seed you can afford. It pays dividends in the long run.

If you don't trust the box stores then go to your local nursery--one that sells all manner of garden stock.
Most of these operations have a department where bulk seed can be purchased. The nurserymen who run the operation ---not the summer help...can usually be relied on to know something about the products they sell.

'Course, no amount of good seed will grow properly on ground that has been poorly prepared to receive it.

To use the car analogy, you can buy a Mercedes or you can buy a VW beetle. Either one is transportation. The Mercedes is a more elite car.

The grass seed you buy in the store is fine for most people in most circumstances. It has more weed seed in it than certified sod quality seed, but there's seed in the soil, too, so buying certified sod quality seed doesn't mean you can't have any weeds sprout, just that they won't come in the seed bag. The main reason some people buy seed from specialty stores is that they want to choose elite cultivars and they want to specify which cultivars they get. The cultivars in the bag from the big box store are going to be cultivars that do reasonably well all over the country (or at least all over the country in places that would grow cool season grasses). If you want grass that has the best green in your area, you'll want to choose the cultivar based on the NTEP trials and choose exactly which seeds you want.

I've always bought the stuff off the shelf, so I've got one of those "decent but not great" lawns. Actually, the past few years, I've been buying from specialty stores, but that's because I'm replacing my lawn with non traditional turf grasses like streambank wheatgrass and western wheatgrass. So my lawn is now much lighter and closer to blue than a lawn that you'd get from the seeds off the shelf. But I'm trying for a lawn that will live in the desert without the water that a KBG lawn requires, so I'll cede the color for the water saving.