The search engine market isn't quite as diverse as it may appear https://www.searchenginemap.com/
There are maybe 4 or so 'crawlers', and the rest buys access to the part of their data they are willing to sell to others.
Running a crawler with the current size and complexity of the internet is expensive, and complicated. Then there is sifting and sorting the data in a reasonable searchable format, and then there is the quality problem, etc.
Much easier to license data access from a provider (Usually Bing or Google or both) and just offer some added features on top, like no tracking, different result UI, custom filtering values per Bing or Google's APIs that make your own "secret sauce", etc.