Hmm, that's a good question and I don't know the answer for sure, but if I had to guess, it would be because writing about the planets, whether what we would now call astrological or astronomical, was almost certainly done in Latin, even in Germanic lands, since that was the language of education and literacy (Anglo-Saxon England had a stronger than usual culture of writing in the "Vernacular" (meaning, the local non-Latin language), so I wonder if they might have some instances of referring to the heavenly bodies by their Germanic names, but I would guess probably not). I'm not positive on this, but I'm pretty sure that the days of the week were introduced much earlier, likely during the initial contact of the Germans and the Romans (German soldiers need to understand their Roman officer's orders, which might include days and times!), and that the planets would have come in later, likely in a more Christian context, where the Roman names for the heavenly bodies had been accepted as "just the names of those things in the sky" rather than "non-Christian Gods" - "translating" them to Germanic God name equivalents might have been a bridge too far for Christian monks.

All of this also points up that we don't know whether or not the ancient Germanic folks equated the Gods that ended up associated with the days of the week with the heavenly bodies of the same name in the Roman system - there's some evidence that Aurvandil/Earendil, for example, was seen as the Morning Star, and so that would seem to imply that the Germans didn't think that heavenly body was Frigg/Freyja. Likewise, Tyr/Tiw, from Whom Tuesday, might have been associated with Polaris, rather than Mars, despite being given "Mars's Day."

Once again, quite tricky and messy, unfortunately, but also again, if anyone else knows more, I'd love to hear it.

Cheers,
Jeff