This is not particularly an android related question. All the phones I have used so far, don't record voice in mp3 format. Is it a limitation of the phones? Is there any benefit in recording voice in other formats than mp3? I really don't get it.
In short, MP3 is a Compressed audio file format, which means some details of Audio will be deleted to reduce the size. If you record your audio in MP3, the quality will be low than recording in Other formats such as ogg or aac.
Hardware is not a limitation. Recorded audio can be converted into any other format using simple software. Note: Now-a-days many applications in Playstore which can record directly in MP3.