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.