Because it allows for maximum flexibility compared to other form factors.
flex ATX above that is the micro ATX
ATX (Advanced Technology eXtended) and Flex ATX are both motherboard form factors, but they differ in size and layout. ATX boards typically measure 305 x 244 mm (12 x 9.6 inches) and offer more expansion slots and features, making them suitable for gaming and high-performance systems. Flex ATX, on the other hand, is a smaller variant at flex ATX measures 229 x 191 mm (9 x 7.5 inches), designed for compact cases while still providing some expandability. This makes Flex ATX ideal for space-constrained builds, such as small form factor PCs.
I'll go ahead and give them all to you by width X depth(length) in inches Flex ATX 9.0 X 7.5 ATX(Full Size) 12 x 9.6 Mini-ATX 11.2 X 8.2 Micro-ATX 9.6 x 9.6
ATX, MicroATX, Flex ATX, BTX, and NLX, In that order.
There are no similar form factors to ATX if you're talking about a slimline. Slimline form factors are NLX and LPX. The form factors to ATX are BTX, Micro-ATX, and Flex-ATX. There is no correlation between slimline and ATX.
FlexATX smaller version of MicroATX
There are not really any advantages with a ATX over a Micro ATX just Micro ATX is smaller and ATX is larger
ATX
I have often wondered what ATX in ATX software stood for. I always just ignored it but upon further inspection I have discovered that the ATX stands for Advanced Technology Extended.
Asus is simply a manufacturer that makes atx and non-atx motherboards. Whether a motherboard is ATX or not is specific to that single model.
Yes, if it is an ATX motherboard. The motherboard specifications should say specifically if it is ATX, mini ATX, BTX, ITX, etc. Most motherboards are ATX.
ATX is the form factor which Motherboards, Computer Cases and Graphics Cards use to standardise sizing. An ATX form-factor Motherboard will fit an ATX Case, and an ATX Graphics Card will fit in the case as well.