Because it's about ownership. The contract documents would outline who owns what component of the fit-out works, where typically (not always), the following would occur:
- Ownership of physical works (e.g. services such as electrical, hydraulics, etc) would be to the Lessor,
- Ownership of loose plant and equipment (e.g. fridges, commercial microwaves, etc) would be to the Lessee
That's why it's better to negotiate (if possible as Lessors are quite savvy) that a large part of the works are under the ownership of the Lessee to maximise their depreciation claim.
Additionally, whomever the Lessee chooses to use to fitout the premises, a cost breakdown per item (e.g. separate the individual finishes, equipment, etc) is advisable rather than a lump sum invoice as specifically breaking it down can reap a larger depreciation amount than applying a single averaged rate on a lump sum.