I suspect that Google finds value in the role they play in the market.
There is demand for search APIs, and companies like Kagi can build a business around that, grow and then compete more generally with Google over time. Serp makes that difficult.
For competitive reasons Google might not want to sell a search API directly (they might indirectly fuel a lot of competition against their main ad supported product). So letting Serp offer this service in a bit of a gray area makes it hard for competitors to form a beach head in search, while giving Google legal flexibility to shut down any service that tries to compete with them in any way through Serp's data.
It's completely fair to compare prices when you're on the buying side.
It's also strongly recommended when you're on the selling side, as you should be prepared to explain what added value justifies that your product is so much more expensive.
Why would I care about that as a customer though? If I compare options, I look at price and performance. If one is making it harder for themselves without any added value for me, why would I pay more money for that?
Given the economics involved (likely a fixed cost per query), I think it makes business sense to try to get few customers to pay a lot to get a lot, much more so than having many customers that pay a little and get results on par Google's free offering.
High end "boutique" search offering a refined search experience (at high computational cost) is a niche Google search can't compete with, since they're offering their search for free, and they'd take massive losses if they drastically increased the amount of compute per query.
It does seem expensive, but if it wasn't, then there'd be more temptation to simply white-label Kagi via it's own API while undercutting Kagi's own plans.
That's actually not so bad. That company reselling Kagi search is also replacing e.g. Kagi support, so there is less pressure on Kagi staff.
And honestly, anything that gets more people into the idea of using not-Google for searching is good for Kagi - even if it is to an ostensibly competitor.
Kind of doesn't make sense to compare to them.