For usability and security reasons.
From the Beginner’s Guide to OAuth:
https://hueniverse.com/beginners-guide-to-oauth-part-iii-security-architecture-e9394f5263b5
… While mostly an artifact of how the OAuth specification evolved, the two-Token design offers some usability and security features which made it worthwhile to stay in the specification. OAuth operates on two channels: a front-channel which is used to engage the User and request authorization, and a back-channel used by the Consumer to directly interact with the Service Provider. By limiting the Access Token to the back-channel, the Token itself remains concealed from the User. This allows the Access Token to carry special meanings and to have a larger size than the front-channel Request Token which is exposed to the User when requesting authorization, and in some cases needs to be manually entered (mobile device or set-top box).
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Note that this question is a dupe of
Why must we “change temporary credentials for token credentials” in OAuth?
If the explanation from the Beginner’s Guide isn’t clear, then go read @npdoty’s take on it .