In this week's Torah reading, Moses calls the heavens and the earth as testimony to the eternal bond between G-d and the Jewish people.

Why not gather ordinary people as witnesses -- a somewhat more normal method than using such seemingly abstract witnesses?

As the commenter Rashi explains, the heavens and earth endure generation after generation.  Rather than using human witnesses who will eventually pass on and the testimony will be lost, the idea is to have an eternal testament.

We see this in reality.  Even many tens of generations from Sinai, not that much has changed.  We still follow the same Torah, the same laws, traditions and customs, preserving this eternal bond and passing it on intact to the next generation.  It is all the more impressive since, as someone remarked recently, G-d spoke to the Jewish people thousands of years ago at Mount Sinai, and He hasn't spoken to them collectively since, yet we still do what He told them to then!