The upfront capital costs and fixed running costs are huge, and the commissions are very low - often zero, normally about $1 or $2, only very rarely $10+ if it’s a high-priced purchase (business/first class with multiple passengers, but that’s rare on a budget flight funding site). And because travel is an infrequent purchase for most people, you don’t get much of a retention flywheel.

This is why there are close to zero flight search engines that are successful standalone companies and almost all the big brands have been acquired by bigger dominant players - Kayak is now a sub-brand of Booking (prev. Priceline) and Skyscanner was acquired by Trip.com in China.

Yes they can be valuable properties but more as a traffic generator for selling other products (accommodation, car rental, insurance), and you need a big team to do all that.