First, professional courtesy. Search was a very small world in 1998 (that is, in terms of people working in the field). There was an academic quality to the industry, with a lot of freshly minted college kids, teachers and universities involved.
Second, a stamp of confidence in their technology. That is: if we're not doing a good enough job, we deserve to lose, and here's our competition.
Third, money. Companies often paid to be listed at the bottom of search engines. In 1998 Google was still highly reliant on partners that used their search technology (they were battling eg Inktomi in this respect).
It's a bit of a shame that the link to AltaVista doesn't point to the search engine site. I think they did eventually get altavista.com (it redirects to Yahoo now) but the IA link given pre-dates that.
