Many sites allow you set an alert, i.e. you get a notification if the price falls.
Anything more than this would not really be helpful. Airline prices these days are NOT apples to apples, i.e. there are many fees for bags, seats reservation, carry on, food, fare class, etc. There can be also massive differences in number of stops, flight time, layover time, etc. Even two tickets at the same or very similar nominal price may present very different value to you depending on your specific needs & preferences.
Automated booking is a non-starter. An airline booking requires quite a bit of information, a lot of which is confidential. It needs to be secure and can't easily be stored by 3rd parties. You are also required to manually accept the terms and conditions of the specific ticket for the specific airline. To give a you flavor: the terms& conditions for a ticket I bought in January had 751 pages. This was just a single itinerary with a single airline.