Because not all OSs are Unix, not all Unixes have sendmail,
configuring sendmail is difficult.

Do you use ed as your editor or something else? ("$ cat > app.rb" for example)

People use gems so that they can get the job done, the same reason
that they use Ruby.

The unix philosophy is not about being hardcore and using things like
sendmail, its about simple tools that to a simple things well and
joining then all together.

It is about being "good enough" rather than doing things the "proper
way". This is the 'MIT approach' vs the 'New Jersey approach'

http://www.xent.com/FoRK-archive/summer96/0591.html