Others specialize in conference papers, technical reports or patents. Some cover multiple document types. Some are limited geographically. Dissertation Abstracts only covers North American and European dissertations. Chemical Abstracts attempts to cover all of the chemical literature.
Few sources do retrospective coverage, i.e. indexing the literature before the index began.
But some do. Many electronic sources don't go back as far as the corresponding printed tools. Sometimes you only need recent years. For example, in biotechnology or particle physics, information dates quickly. Not so in synthesis, taxonomy, geology, math.