Sure, there are central indexes. There are also lists of the different indexes. I expect there are bibliographical overviews of the lists of indexes, too... and of course, there's a site which will give you the best of all the others...
Many users manage with one or two - you get the addresses with your subscription and software.
The weaver of a web-site puts in keywords, titles and a basic description: he can then rely on the search-engines' robots to find his new pages during their eternal search through the whole web; or he can instead report his site himself to as many as he can be bothered of the several hundred search engines - or pay someone else to do it for him...
But the quantity is inconceivable - the number of web-sites doubled during the last three years, and every day 50,000 - 100,000 new e-mail addresses. are established.. so we have to live with not always finding what we're looking for.
I guess I'll never forget my first serious web-search, where I asked for 'trains in Norway'; I got
16,782 'hits', of which no.5 in order of relevance, was 'bicycling in Denmark' - so what on earth were all the others?!