I've no idea if they considered barrier to entry or relative size when drafting the law or built in any limitations. Of course until an engine reaches some level of popularity it's unlikely to receive many or any removal requests, so should remain manageable for them. If it were the individual sites every site would need a policy and takedown request, just like most have now added something for DMCA requests. Doesn't that just put the barrier to entry up for everyone publishing a site?
And yes, the DMCA is also a barrier to entry for search engines, but at least it's one that I understand well.
Thus, this is one of those times where the lawmakers actually looked at technology, and understood how it was used.
Also, making search engines responsible creates a far smaller surface area of contact than having every site that contains the content.
Also, you might want to consider the barrier to entry for a new search engine.