Don't get me wrong, carriers have been making strides to lower the amount of spam that's sent through the air (A2P requirements, toll-free number verification requirements, etc), but a determined scammer can still exploit SMS/MMS pretty easily.
Most people will trust a toll-free number just as much as a shortcode, and since tons of legitimate companies use toll-free numbers for messaging it just blurs the line of what a "reputable" number looks like.
Even SendGrid, which is owned by Twilio, uses toll-free numbers for their 2FA messages instead of shortcodes.
As far as I could tell (although I retired in 2019, so might be out of date), you can't use one short code through multiple aggregators, so if you want the benefits of multiple routes, you've got to have multiple shortcodes or live with sending from regular phone numbers.