If you're making Mac & cheese from scratch, why not just use real cheese? Or multiple cheeses (and herbs and...)?
Aside: A roux is the basis of many 'creamy' sauces. Learning the skill of roux making is very much worth your time as a home cook.
If you still can't manage to get it down, try xantham gum instead. Mix it in with some oil first, then add your liquids. Again, you'll need to experiment with hydration ratios, but it's a much easier and faster method that will also work for cold dishes. It's powerful stuff, maybe a teaspoon per liter of water will thicken quite a bit.
And unless you're absolutely blasting the heat, in which case most food behaves the same, there's no way for a roux to go from undercooked to burnt in a few seconds. It can take an hour to make a dark roux for gumbo.
Sodium citrate for example helps your cheese melt creamily.
The powder likely contains the right balance of additions that give you a great creamy mac & cheese.
A good reference: https://www.seriouseats.com/recipes/2016/11/sodium-citrate-b...
Eh, is this a US thing? Here in the UK, I just use cheddar - any brand, any maturity you fancy, and it melts just fine.
I spent several months in the US around 2005, and I do recall the cheese was pretty terrible - bland, with an odd, plasticy texture.
Aw, dude, you're supposed to remove the individual plastic wrap from the cheese before serving!
In Britain they label it "singles" or "slices" or something equally meaningless.
They do seem able to use "cheese" when English isn't a national language, though they then can't use the local word for cheese.
They are called "pasteurized process cheese food" or "pasteurized prepared cheese product" depending on their ingredients. There are pretty specific FDA regulations on cheese naming that are presumably similar to the equivalents in the EU: https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFR...
Another Serious Eats reference. https://aht.seriouseats.com/2011/07/the-burger-lab-how-to-ma...
May as well ask, why not curdle the cheese yourself?
Why buy yogurt when you can make it yourself?
Why buy salad dressing when you can make it yourself?
Why buy soy sauce when you can make it yourself?
Why buy hot sauce when you can make it yourself?
Why buy canned food when you could can it yourself?
Why buy soy milk when you can make it yourself?
Why buy salsa when you can make it yourself?
Why buy mayonnaise when you can make it yourself?
Why buy tomato sauce when you can make it yourself?
Why buy soup mix when you can make it yourself?
Etc.
It's a matter of personal taste, cost, convenience, time, resources, skill level, shelf life, day-to-day diet, etc.
Also, a sibling reply mentions roux. I haven't got that option myself because I'm allergic to wheat, so I'm personally stuck with "alternatives" regardless.
Cheese powder, on the other hand, is non-perishable so it can always be kept on hand. It's also MUCH cheaper.