Say your application and database are in the US West and you want to serve traffic to EU or AUS, or even US East. Then you want to terminate TCP and TLS in those regions to cut down on handshake latency, slow start time, etc. Your reverse proxy can then use persistent TLS connections back to the origin so that those connection startup costs are amortized away. Something like nginx can pretty easily proxy like 10+ Gb/s of traffic and 10s of thousands of requests per second on a couple low power cores, so it's relatively cheap to do this.

Lots of application frameworks also just don't bother to have a super high performance path for static/cached assets because there's off-the-shelf software that does that already: caching reverse proxies.