How so? Signal supports Android and has a cross-platform desktop app (that supports not only macOS, but also Linux and Windows), while iMessage does neither. Signal also has perfect forward secrecy, while iMessage does not.[1] Signal even allows everyone to create and publish sticker packs free of charge,[2] while iMessage requires a $99/year Apple Developer subscription to do the same.[3] I suppose Signal doesn't let you create Animoji or Memoji, but that's not essential to me.

[1] https://eprint.iacr.org/2021/498 (page 19)

[2] https://support.signal.org/hc/en-us/articles/360031836512-St...

[3] https://www.theverge.com/tech/2016/9/23/13005160/imessage-st...