There's no obvious network effect in search.

Google does have brand and distribution though.

On targeted advertising the barrier is that you must already have a significant fraction of the personal data Google has before you can start to compete.

Not in search, in ads. The ad network itself. Many many many sites integrated Google Ads (AdWords, AdSense, Adwhatever).

Plus the sheer volume and reach that Google has. (It pays millions - if not billions summed up for every little channel they pay for - every year to be the de facto "internet" for many users, many kinds of devices.)

It's basically unimaginably hard to compete with this for newcomers. Sure, they can carve out a niche, and so on, but that's just that.