Because mass is not required in order to be affected by gravity. Your high school physics is just a first approximation of how the world works in many, many ways. Einstein’s field equations, which tell us how some distribution of energy throughout spacetime creates gravitational effects, involves 16 equations. Newton’s law of universal gravitation is just one term of one of those equations. It just happens that in most of the situations you are interested in, it’s the only term that is significant. That’s why Newton’s theory works so well. But, those other terms and those other equations are there, and they influence things. Light’s response to gravity comes in that way.
You were also taught in high school that momentum is mass times velocity. But light also carries momentum, even though it has no mass. More generally, you were taught that all those equations you learned were exact and precisely predicted the future. They do not - they are, in fact, averages over quantum theory. In most everyday world situations they are so close to exact that you can regard them as exact. But they are not.
Keep learning - you’ll get there.
Stay safe and well!
Kip
https://www.quora.com/If-light-has-no-mass-why-is-it-affected-by-gravity-3
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