One might also ask, "Why isn't the Mail app green like the other communications icons?"
Of the four standard Apple apps used for two-way communication, three - Phone, iMessage, FaceTime - are green, while
Mail is blue.
I read a news story a little while ago where the green color-coding for SMS messages was referred to as Apple "treating Android users as second-class citizens." Really? What's second-class about green? It's color-coding, because it's useful in a app supporting multiple communications protocols to know which messaging system is being used, SMS or iMessage. It's helpful when troubleshooting to determine which system may not be working.
And yes, SMS is feature-light relative to iMessage (and other internet-delivered messaging systems) - SMS is an industry standard, and you know what they say about things designed/administered to by committee. SMS has often been treated as a revenue source by the telcos. Remember when US telcos charged $0.25 per text, and per-megabyte for cellular data? Some telcos continue to treat some aspects of SMS as optional revenue-generating services (MMS, group messaging), holding them hostage for premium service plans.
Apple certainly has wanted users to distinguish between iMessage and SMS. Originally, before all the Animoji and "fun" features, iMessage was a money-saver (see previous paragraph) - a free benefit of staying within the Apple ecosystem, available over cheap wifi rather than expensive cellular data, available on non-cellular devices (of which Apple sells more than a few).
If Apple wanted users to truly discriminate against SMS they'd have included an ON/OFF toggle for SMS. Instead, it's effectively the mandatory messaging system - as long as you have cellular service you'll have SMS. This is appropriate, of course, due to SMS's role as a worldwide communication standard.