Why isn't a defendant entitled to an attorney if a lineup is conducted before the person is arrested?

Simply put, if you are not yet charged, or more properly
put,not yet arrested, then your 6th Amendment rights do not apply. It is only after
you've been charged that your right to counsel kicks in. Indeed,if I correctly
understand your premise, then the person in the line-up is not under arrest, and is free
to refuse to take part in the line-up. He may leave.

If he
is in fact under arrest, then he should be represented by counsel at any line-up.
Failure to honor that right will almost certainly result in the line-up results being
suppressed.

One last note: Miranda is not required if the
person is not in custody.