**Yes—some services advertise exactly this**, although automatic buying is less common than price alerts. What you describe is essentially a flight *limit order*.

| Service | What it offers |
|---|---|
| [SetMyFare](https://setmyfare.co.uk/how-it-works.php) | Says you select a flight, set your target price and authorize payment. It checks every 15 minutes and automatically books and charges your card when the target is reached. |
| [AirScalp](https://airscalp.com/pricing/) | Advertises target prices, itinerary constraints and optional automatic booking attempts. It is a **beta/early-access service**, listed at $99/year, and requires your own supported flight-data API key. |
| [Milo](https://milo.flights/) | Lets you specify a target price, airports, date window, trip length, stops and airlines. **It alerts you; you make the purchase.** |

Those automatic-booking descriptions are the providers’ own claims; I haven’t tested a purchase or independently established their reliability.

As for why this isn’t standard everywhere, **the evidence points to practical and industry hurdles rather than a special prohibition on target-price purchases**:

- **A search price needs confirmation.** Amadeus’s booking workflow explicitly checks the latest price and availability before booking. A service finding a fare below your threshold still has to secure it before it disappears. [Amadeus documentation](https://github.com/amadeus4dev/developer-guides/blob/master/docs/resources/flights.md)
- **“Meets my criteria” can be complicated.** Baggage, refundability, connections, airports and departure times all matter. My inference is that automatic buying needs much tighter instructions than an alert does; otherwise it can buy a cheap ticket you wouldn’t actually choose. [DOT’s explanation of ticket differences](https://www.transportation.gov/individuals/aviation-consumer-protection/buying-ticket)
- **Issuing tickets adds operational obligations.** For example, Amadeus requires production booking customers to meet local legal requirements and work with a ticketing consolidator. Changes and refunds then require support through that consolidator. [Amadeus booking FAQ](https://admin.developers.amadeus.com/self-service/apis-docs/guides/developer-guides/faq/)
- **Some major search sites don’t sell the ticket.** KAYAK identifies itself as a search engine rather than a booking provider, so adding automatic purchase would extend its role considerably. [KAYAK help](https://www.kayak.com/c/help/pricing/)

Your idea is feasible. The critical feature to look for is a **hard maximum on the final total**, including required fees and baggage, alongside precise itinerary constraints—and clear handling when the fare changes before booking.