Low quality SMS. There are lots of things Apple could do to improve the experience of texting people without iMessage, lots of things built into the SMS standard that they do t implement.
Edit: wow thought this was commonly known. Basically Apple hasn’t adopted industry standard SMS improvements. There’s a whole campaign to try to get them to. Here’s an article explaining https://www.android.com/get-the-message/
Welp, glad everyone isn’t like you. The world would be much worse.