[By whom?]
"The physics prize has mostly consisted of string theorists giving it to their friends in the field."
Firsty, the Breakthrough Prize is given in Life Sciences, Mathematics and Fundamental Physics. This post is about the Breakthrough Prize in Mathematics.
As for Fundamental Physics, apart from 2019 (special) for supergravity, it doesn't seem to go to string theorists:
2021 Special Breakthrough Prize In Fundamental Physics -- Steven Weinberg, “continuous leadership in fundamental physics, with broad impact across particle physics, gravity and cosmology, and for communicating science to a wider audience.” [0]
2021 Breakthrough Prize in Fundamental Physics -- Eric Adelberger, Jens H. Gundlach and Blayne Heckel, University of Washington. Citation: For precision fundamental measurements that test our understanding of gravity, probe the nature of dark energy, and establish limits on couplings to dark matter. [1]
2020 Breakthrough Prize in Fundamental Physics was awarded to all 347 members of the Event Horizon Telescope Collaboration representing 60 institutions in 20 countries. [2]
2019 Breakthrough Prize In Fundamental Physics -- Charles Kane and Eugene Mele – University of Pennsylvania. Citation: For new ideas about topology and symmetry in physics, leading to the prediction of a new class of materials that conduct electricity only on their surface. [3]
2019 Special Breakthrough Prize In Fundamental Physics -- Jocelyn Bell Burnell – University of Dundee and University of Oxford. Citation: For fundamental contributions to the discovery of pulsars, and a lifetime of inspiring leadership in the scientific community. [3]
2018 Breakthrough Prize In Fundamental Physics -- WMAP experimental team. Citation: for detailed maps of the early universe that greatly improved our knowledge of the evolution of the cosmos and the fluctuations that seeded the formation of galaxies. [4]
There are some string theorists further in the past, but they still don't warrant the use of the word 'mostly'. [5]
[0] https://breakthroughprize.org/News/61
[1] https://breakthroughprize.org/News/60
[2] https://breakthroughprize.org/News/54
[3] https://breakthroughprize.org/News/47
[4] https://breakthroughprize.org/News/41
[5] https://en.wikipedia.org/wiki/Breakthrough_Prize_in_Fundamen...