Why shouldn't applicants to job openings send their cover letter as an attachment in an e-mail? when would it be acceptable
to send a cover letter as an attachment?

To grab the attention of the hiring manager, applicant has to give the synopsis of his skill set in covering letter in the body the email.  

Explanation:

The hiring managers are overloaded with lot emails to recruit the potential employees. So the applicant has to grab the attention of the hiring manger, moment he opens the email. To make that, the applicant has to design the body of email in such a way that the applicant has to express his skill set and job expertise in the area of the recruitment.

So it will instigate hiring manager’s curiosity to open applicant’s resume for further information. This way the applicant has concentrate on the body of email to grab attention of the hiring manager, while sending the resume and covering letter as attachment.

for submitting your question to

the answer to your question is : c) a government agency or homeowner

due to his line of work, he is most likely employed by a government agency or homeowner due to the other choices not being logical haha.