Reasons:
- Less DNS seeds we trust from devs less active right now, better for us especially people involved involved in misleading users
- 1 DNS seed removed from Core wont't be a big impact: No PR since Since 2019 or reviews. Reviews on privacy related PRs would have been useful but updates about privacy were only on Twitter for some reason, it doesn't help Bitcoin development and the activity in soft fork combines with misinformation doesn't looks good.
If removing feels rude, there should be a process which I am not aware of. There was a recent question and I think new nodes can be exploited with DNS seeds: https://bitcoin.stackexchange.com/a/104923/