The real reason: Apple intentionally doesn’t support the open protocols that send pics and videos to non-Apple devices. These protocols are a decade old and work great. They use a proprietary protocol instead, which they will not share with other phone manufacturers.
What the average iPhone user thinks: Apple is better than Android!
It’s pretty dumb.
Unless I did a really poor job researching it, you cannot change your default SMS/MMS application on an iPhone.
You can use other messaging apps like Signal, Whatsapp, Telegram, or AIM. But if you want to use SMS, you have to use iMessage.
Maybe this is US-specific though. Europe often forces Apple to do things they don’t do here.