The dry spaghetti from Barilla really sticks for me if it's been more than a minute since I drained it

These fuckers dont salt their water. Thats literally it.

IT SHOULD BE SALTY AS THE SEA. YOUR PASTA WILL NOT STICK.

Conversely you don't need to use nearly as much water if you are willing to just stir it a couple of times during the short period of time it takes to come back to a boil after you add the pasta.

Less energy used, less salt needed to get the water properly salty, less time to get the water up to a boil, are why I choose that method. It comes out the same in the end, both methods are valid.

I don't have this issue when making spaghetti, but I do have this issue when making a box of mac and cheese for my kid. I use way less water for the latter, but it's the amount of water they specify on the box.

The way I do it, the boil for both is usually not a rolling boil; the water has just started to bubble and steam. I started doing it this way years ago when someone told me that noodles typically cook (the starches gelatinize, I think) at a temperature just below boiling, so you don't actually need a full rolling boil to fully cook pasta.

I salt the pasta water for spaghetti, but not for the mac and cheese.

If I don't stir the mac and cheese regularly, it will aggressively stick together. The spaghetti will eventually stick together, but takes waaay longer. This never happens in practice because I manually stir every few minutes while I'm making the sauce.

People either don't have enough water or have the heat too low. Enough water will give the pasta space and enough heat will agitate it.

Now that I'm here, also put a lot of salt in ypur pasta water. It should be as salty as the sea. If you make a sauce, the pasta water is a good start, the starch will make it sticky. Source: Samin Nosrat.

I use barely enough water to cover the pasta. (Bonus: the water used helps make a very thick sauce!)

I don't have this problem because I add a tiny spot (maybe 2 ml?) of oil to the water.