Why doesn't the BFR/Starship have a launch escape system?

You are not the first to raise this question! It's worth reviewing Tim Dodd's accessible and detailed evaluation of escape systems: https://everydayastronaut.com/starship-abort/

He draws attention to a number of considerations:

• escape systems are not a panacea.

So in the grand scheme of things, to date, a mechanical abort system has only saved lives twice, may have prevented one tragedy and in one case caused a death. So out of the 320 orbital human flights to date, only three missions in total necessitated the use of an abort system, or less than 1% of crewed launches.

(And to paraphrase one commenter: if our concern is around the reliability and stability of rockets, it is ironic that our solution is "lots more rockets!". There are significant concerns about a) putting powerful, hair-trigger rockets and their fuel right next to the passengers, b) difficulty of testing, c) risks around carrying said rockets back through re-entry.)

• the Challenger Shuttle disaster fatalities were caused by a series of failures at many levels.

perhaps the biggest problem with the Challenger disaster wasn’t a hardware problem, but a problem with program management and pressure to get that flight off the ground. It was known that they would be launching outside of the predetermined operating envelope of the SRBs and it was recommended to not launch that day.

(Of course, it doesn't matter what the cause of the failure is if you need to escape! But the big lessons lay elsewhere.)

• trust is earned.

So that’s why I think it’s vital we see these things fly, fly often, and fly over and over. Only then will I think there’s a proven reliability record that would make it a safe enough option to not have an abort system.

A point that I think is rarely made, is that commercial airliners don't need escape systems because we can perform acceptance tests on them. Airliners are taken on test/commissioning flights before and during delivery to their customers, where their airworthiness is evaluated and certified.

You can't do this with disposable rockets: every flight is a maiden flight! And this doesn't exclude the Space Shuttle here, as the extensive refurbishment required after each flight would absolutely qualify for a test flight, if it were airliner, before passengers were allowed on board.

But with cheaply reusable and autonomous rockets, it's a completely different story. By specifically targeting low-maintenance and minimal refurbishment, SpaceX can do test flights and cargo flights until all the hardware and procedural issues are ironed out and the Starships can launch with an airline level of confidence. It is this capability that is unprecedented, and which is unsettling the established view of launch safety. Of course, they may fail in this goal, but that's why the whole grand saga has become so exciting lately!