Why are there ancient, long extinct scripts (e.g. cuneiform) in Unicode?

I’m going to put in a less popular answer:

Because they can.

Yes, there is research ongoing on extinct scripts, and scholars should be able to exchange texts in those scripts. The thing is, scholars usually exchange Sumerian, Old Egyptian, Mayan etc texts not in the original scripts, but in transliteration. The scholars are consulted in putting together the Unicode representations of their scripts, but they are not, from what I have seen, desperate to see them adopted because their absence was blocking them doing their work.

You can’t rule out that someone will want to use them, even if just in illustratory text, and you do occasionally see old scripts used as plaintext by scholars (Egyptian hieroglyphics more than cuneiform, cuneiform more than Mayan hieroglyphics). And Unicode is intended to be the definitive encoding of all scripts that could ever be digitised. So their presence in Unicode is legitimate; but it was never pressing even within specialised fields. That’s why they got bumped to the “Astral Plane” (the Supplementary Multilingual Plane, U+10000 to U+1FFFF.)