That's only because they're terrified of him. From what I remember of the books, Lockhart is arguably a better teacher. Snape doesn't teach anything, just puts the directions on the board and goes around glaring, snapping, and psychologically sabotaging his students. Lockhart at least tries to do something, even if he's useless. The one thing Snape has in his favor is knowledge, both in his head and in the textbooks.
Eh... On the 'teaching' side of things, Snape does send and correct essays, so the kids
do have some idea of what each ingredient does before they try to use it. Not like they'll turn in a blank essay to
Snape, of all teachers.
But I'd say Snape's job is more about keeping the students alive than anything else.
Would you like to be the only adult on charge in a room with 40 to 80 eleven years olds using dangerous substances? Explosive substances? Toxic substances? Corrosive substances? That all work magically so you can only barely predict how the kids are gonna mess up themselves and others if you're a Master at the subject?
I've never seen a chemistry lab without all the safety equipment and at least 3 people overseeing things. And that's for 20, maybe 30 people, adults or close to adults.
Now think about how the House 'rivalries' and the fact that the classes are mixed mean the Slytherins and the Gryffindors attempt to 'sabotage' each other by throwing random ingredients into each others potions.
Really, even if Snape hadn't been such an asshole as a youth, I wouldn't blame him for being one
now. Mind, Slughorn seemed to do OK, though I don't quite remember just why he didn't really want to go back...
At the end of the day, Snape's
obvious deficiencies (bullying kids, thought that fit quite well with the themes before HP tried to 'grow up') aside, he's always caught potions 'accidents' right after they happened. Most of the time he's in class he's thinking of all the ways things can and will explode and ready to take care of it before it gets too bad.