So many products are like this - it sounds good on paper to consolidate a bunch of tasks in one place but it's not without costs and the benefit is just not very high.
If they become popular they'll have to move to OSM, Google's steep charging for their Maps API at high usage that has brought companies to their knees is well known [1].
For now, they do use Google Maps and I’m happy with it. If they stop and it’s no longer as useful to me, maybe I’ll stop using it.
But I use it as a glorified notes app to keep track of flights, reservations, rental cars, confirmation numbers, etc, in one place, not for trip planning.
Similar to "made for everyone" social networks and video upload platforms.
But there are niches that are trip planning + there are no one solving the pain! For example Geocaching. I always dreamed about an easy way to plan Geocaching routes to travel and find interesting caches on the way. Currently you gotta filter them out and then eyeball the map what seems to be nearby, despite there, maybe, not being any real roads there, or the cache is probably maybe actually lost or has to be accessed at specific time of day.
So... No one wants apps that are already solved + boring.
1) A third party app simply cannot compete with Google Maps on coverage, accuracy and being up to date. Yes, there are APIs you can use to access this, but they're expensive and limited, which leads us to the second problem:
2) You can't make money off them. Nobody will pay to use your app (because there's so much free competition), and the monetization opportunities are very limited. It's too late in the flow to sell flights, you can't compete with Booking etc for hotel search, and big ticket attractions don't pay commissions for referrals. That leaves you with referrals for tours, but people who pay for tours are not the ones trying to DIY their trip planning in the first place.