Probably the same way as e-mail spam/scam. When you reply, they know
they hit a valid and monitored e-mail account. The same for mass puking
out to phone numbers to send untargeted and bogus texts. When someone
rings your doorbell, you could get quiet, hide, and they don't know if
you're home or not, but once you answer the door then they know you're
there. Spammers and scammers much prefer using hit lists of active and
monitored e-mail addresses and phone numbers than puking out to random
e-mail addresses and phone numbers. Active and monitored accounts are a
coveted resource to spammers.
Anyone that sends me an MMS message is a spammer. My friends, family,
doctors, pharmacy, and everyone else I know send just regular SMS
messages. It's the spammers that use MMS to me. I report them as spam
and block them (a function in the Google Messages app), and obviously
never open the MMS message. If someone can't send me a message within
160 characters (70 with an emoji - but I don't chat with children
enamored with happy faces), has to use MMS to send images and GIFs, and
a message up to 1600 characters, those are senders I don't want to
contact nor see their glitzy oversized message. To me, texts are short,
and not to be some book for me to wade through with pretty pics or
animations. If they cannot compose their message in a short text, they
should be calling me to have a conversation, or e-mailing me. Every MMS
that I've received has been spam.
Hmm, I wonder: can MMS be disabled on my phone? I can disable automatic
receive of MMS, but I still get the notification blurbs telling me
someone sent me an MMS message prompting me to accept/get or not.
Apparently I cannot block multimedia messages unless I block the sender,
and I'm not filling up my blacklist with every asshole spammer sending
me MMS texts. In the Google Messages app's settings, I already have
"Auto-download MMS" disabled. I'd really prefer to block any and all
MMS messages. If SMS, accept. If MMS, kill.