There are things they consider difficult. The building is responsible for all tenants in the building. Things like key card access to the building after hours, key card access to the parking structure, etc are all handled by the building. If the shared office space tenant is constantly asking management to revoke keys, issue new keys, etc, that becomes a pain point for them. In my experience, it was the key card access that was a huge problem. Also, each new tenant usually is expected to fill out some sort of credit/background check, and those cost money. Shared office space companies can have very high turn over, so each new sub-tenant would have to go through that process as well. The building management is going to want their company they trust to do it rather than having the shared space company handle it on their end. Sure, they can request the fees be re-reimbursed, but they still have to spend time/effort on it.
So imagine all of the things you have to do for a new lease. Multiply that by the numbers in a shared office space. People like doing the least amount possible. CBRE managed buildings prefer to have long term leases, so they only have to do this at the beginning of a lease, not constantly.