Why aircraft remain useful launch platforms for hypersonic weapons

Would hypersonic missiles be better confined to cruisers and destroyers rather than mounted on aircraft?

No. Air launch can extend reach and give the weapon the altitude and initial speed otherwise supplied by a separate booster stage.

At least some hypersonic weapons are likely to be adapted for aircraft carriage. A strike aircraft could move the launch point, approach from another direction, and carry the weapon closer to its useful flight regime. Starting high and fast reduces the work needed from a booster before a hypersonic propulsion system can operate. An F-35 might carry only one or two externally, but the aircraft would still add operational flexibility that a fixed shipboard cell cannot.