Sendmail doesn't deliver mail to the local mail spool; this is handled by the local delivery agent (LDA). Look for
grep ^Mlocal /etc/mail/sendmail.cf
The P=xxx parameter is the program that is called to deliver the mail locally. If it is procmail, then you can change the /etc/procmailrc variable DEFAULT to be the user's home directory/desired_mailbox_spool.
See man procmail(1) for more information on setting DEFAULT.