Because I can leverage that knowledge of physics and chemistry to build something useful - and actually do so in my job. I use that knowledge to produce good things for customers, feel the satisfaction of that connection between my study and what I can offer people, and now that I'm writing about it, have a thought that I haven't had in a long time that my grandfather would indeed approve (so double thanks for the question!).
Chess may be more satisfying, but as a career, it's pretty self-satisfying, and only the very top competitors even really provide entertainment for others. That said, chess can teach you a lot about strategy, your own mind, managing your own emotions and skills, etc., and all of those are useful skills to apply to life in general (as long as you don't fall to the addictive risk and make a burnout career out of it...).
Chess may be more satisfying, but as a career, it's pretty self-satisfying, and only the very top competitors even really provide entertainment for others. That said, chess can teach you a lot about strategy, your own mind, managing your own emotions and skills, etc., and all of those are useful skills to apply to life in general (as long as you don't fall to the addictive risk and make a burnout career out of it...).