Really appreciate this- it's been on my mind for some time. I've been off the convention circuit since COVID hit, but before then I found myself awash in lanyards, stickers, pamphlets, t-shirts, plastic and canvas bags, etc. and it did weigh on my conscience.
I think some of this is purely cultural, insofar as startup tech companies see other startup tech companies do this and want to emulate as a kind of legitimacy-builder. I can attest that making custom shirts/hoodies for your team can actually be more fun and morale boosting than it might seem before you do it.
It's very cheap to produce merch compared to the money you might spend on conference attendance, which is another early sales/marketing driver for young companies- if you're spending tens (or in some cases hundreds) of thousands of dollars setting up booths or flying internationally to be somewhere, what's another couple of hundred to give your logo a bit more visibility?
One thing which isn't widely known is that some popular alternatives to swag i.e. food or coffee giveaways are often strictly controlled/monopolised by convention centres- so you'll often be forced to use their services, which can run into many thousands of dollars.
From my perspective as an attendee, the disposables do get really tiresome. I've joined the ranks of the business-card-free and don't think I'll move back. I'd like to see more eco-friendly manufacturing for this space, which probably starts with moving away from plastics and towards more paper. I do appreciate T-shirts and hoodies I've collected and wear even the old ones to this day.