Why are fard prayers shortened but sunnah prayers not shortened while travelling (when a person is musafir)?

We perform four-rak'ah fard prayers as two rak'ahs whenwhile travelling (when we are musafir) but there is no change in sunnah prayers. We perform them in the same way. Why are sunnah prayers not shortened though fard prayers are shortened?

Dear Brother / Sister,

Fard prayers have to be performed; they are shortened while travelling to provide ease. However, there is no obligation for sunnah prayers. If a person has difficulty in performing sunnah prayers while travelling, he can abandon them. If there is no problem, he performs them as four rak'ahs.

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