There is no code requirement for handles themselves AFAIK. Some municipalities require unimpeded egress per IBC 311 (if adopted and not amended). IOW locks that do not require keys (e.g. double keyed dead bolts) or special treatment to exit. The reason you have the door knobs you do is likely they were cheap when the builder bought 10,000 of them. Could also be that the inside handles were not available in a finish that would last on the exterior.
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