Because:

1) ABS has a higher glass point and heat deflection temperature than regular PLA
2) Is less brittle than regular PLA
3) ABS can be smoothed safely with acetone (a substance your body produces itself, relatively harmless), also makes it 100% water-tight
4) Costs less than half of what the 'performance' PLA variants that try to come close to ABS as to 1) and 2) cost... ?
5) Prints with very fine detail and without any stringing. PLA and PETG tend to string more or need much more precise calibration to prevent it.

As to point 1) and 2), even ABS does not cut it for a lot of my applications. Hence now printing a lot with polycarbonate and now exploring PEI filaments....

As has been remarked before, there are a lot of applications in which any PLA or PETG simply does not meet the specs.