Not a stupid question at all. We actually considered this option, but OpenPGP.js looked pretty bad back then.
Security-wise the library wasn't in good shape. One of our cryptographers would "classify [OpenPGP.js] as trash". It has been audited recently, but the result doesn't look very good either [1]. I don't know the current status though.
OpenPGP.js didn't actually implement most of the ciphers - it just imported them from various sources. This made the library inconsistent, i.e., some functions expect string, while others expect byte array, which in turn made it harder to use correctly in a language like Javascript. If we chose OpenPGP.js, we needed to change these ciphers anyway, so we thought it's just better to write them from scratch.