Because no device sends pictures as SMS, not even an iPhone. An iPhone uses iMessage, a server based messaging system like Facebook messenger. Basically it uses data instead of SMS to send pictures. If the recipient is an iOS device, it will use this, but if the recipient is an Android device, it will receive it as an MMS which creates a very compressed picture.
Android does not do this because of decisions it had to to enter the game several years back. Back then, it can't afford to alienate carriers so Google can't create an iMessage-like service built into the phone. You had to download it, (Allo/Hangouts), which means the other person also needed to download it.
As of now, Android is moving to use Google RCS which allows to send full resolution pictures through the carriers. It's basically the next generation of SMS. However, AFAIK not all carriers in the US has implemented it yet.