Black and white thinking. Of course NAMM isn't the only way for companies to advertise and make connections. But it is one way. And for many companies a worthwhile investment of time and money to participate in. For others a work related activity that persists irrespective of its value.
Ever have a job in which you get a certain budget for pro development? Get to fly/drive somewhere, stay in a hotel, go out for some meals with some friends or colleagues, attend a conference for a couple days, get out of the office and daily routine. You sit through some workshops, get some swag, hear a bunch of presentations that tell you stuff you've mostly heard before. You go home, not much changes as a result of info you probably could have gotten had you ordered a book on Amazon. But you keep going each year. Why? Because doing something out of the ordinary, even if expensive is fun. Same reason people spend enormous amounts of time and money planning to go on far away vacations to sit around and do the same things they could do within an hour of where they live.
For the big dogs, like Gibson, it's understandable for them to skip. As they have that rare iconic branding that ensures they will always have market share and significance even if less known companies can put out products that surpass theirs in quality for less cost.