Why hardglass halogen capsules have an exhaust tip like their quartz counterpart?

Each lamp has to have an exhaust tip somewhere.
When the bulb is of a large volume and thin lead in wires passing the wire seal, some extra volume is not a problem, it becomes possible to hide that tip e.g. into the socket or base.
But when the capsule is small with rather large seal structure (thick lead in wires,...) and temperature sensitive (like the halogen capsules are), there is usually no other choice than leaving the tip on the top or on an opposite side than the seals are, so it remains visible.
It does not matter if it is quartz or hard glass, the limitations are practically the same...

Quite correct, but some hardglass halogen capsules do have a partly hidden tip between their leadwires in the base area.  These were developed for automotive uses, where a top tip would create optical distortions.  Another reason why this was not always done is because every part of a halogen lamp needs to be above a certain minimum temperature to keep the halogen cycle running.  If the tip was in the base and too long, it would run too cool and cause the halogen cycle to stop.