Actually, they're not. Or at least, not what I would call 'search engines'.
They're the front end of a group of search algorithms that link back to a carefully-cultivated database of information about the Internet. They're like the cutting edge of a knife, or the point of a screw — definitely the business end, but unable to exist usefully unless the rest of the machine is wielded correctly.
A true search engine is one that generates searches, just as an internal combustion engine is one that generates internal combustion, and a calculating engine is one that generates calculations. Similarly, those searches can be used to power other tasks through clever linkages and channels. This is true of internal combustion and calculation as well.
An engine, therefore, is a producer of power that is the result of processing basic material. It is still useless unless the output is connected to something that can make use of the output.
So what is Google then, if not a search engine?
You, dear reader, are the search engine. Or at least, the meat casing around the search engine that is your brain and nervous system. You generate the searches. You sift the findings. You use the search results by interfacing with the real world. Google is only the carburettor, the feed that you control as you search.
The consequences of not knowing this are dire. I have many students who don't know how to use Google and its huge Swiss-Army-Knife-style adjunct collection of tools. They think you just key something in, without taking time to think about what makes the best query, and get results.
Well, yes, you do. But not often the most useful or the best in quality. You get advertisements and awkward pieces of irrelevant stuff, unless you know how to refine your search. If you can do a search that yields fewer than 400 hits, this is extremely useful, since you won't have to trawl through so much junk.
Doing a properly designed query is like filtering the fuel and air input before combustion. It makes for smoother performance. So too does using the right mixture of fuel and air, in the right doses.
This is what I have to teach the students of today. I suspect that the majority of students these days are extremely test-savvy and terribly information-dumb. Information, to them, is connected to 'technology', but not to 'brain' or 'cognition'. How sad.