Because you have the full power of JS available to do whatever you want, however you want, to generate HTML. With a templating engine, you only have whatever the templating engine provides.
React would let you use JS-powered templates.
Angular would let your use ANY (static html, JS generated, or dynamic/server generated), because its pluggable.
For me, the power offered by Razor(cshtml) is too much to abondon.
