Why doesn't the activity "send email" have a 'schedule follow up activity' option
Great question, and you're not missing anything: that feature just doesn't happen to exist at the moment.  It would be both useful and consistent with other activities.

With CiviCRM (and many other mature open-source projects), there often isn't a grand product plan, or if there is, many features come from people solving specific problems they or their clients have.  When the email feature was added, someone either didn't think about follow-up activities or decided it would be too much work.

On the flip side of this, however, it also means that there's nothing really stopping you or anyone from building that feature except for the time and expertise to write the code.  If you, a coworker, or a consultant can build it, I'm sure the core team would incorporate it into an upcoming version. You could also persuade them to build it by raising a ruckus and/or funds.

Unfortunately, this is one of the few parts of open-source where you really get what you pay for: if nobody shared the feature, it won't be there, no matter how logical it would be.