Because, like the Oscars, they're industry peers recognizing the work of other industry peers. The people who do the same work telling you your work is the best is one of the highest possible forms of validation and compliment in creative work. TGA is an important PR/marketing opportunity, but that's a very different kind of "matter." I should probably have said "meaningful" instead.
It's the difference between winning Best Actor at the Oscars and winning Best Actor at the MTV Movie Awards. Both are good things, but guess which one's going on the mantel?
The problem is nobody really cares about the presenters to begin with. It doesn't really matter if Pitbull or Tim Schafer shows up presenting the award for Game of the Year. People tune into The Game Awards for the announcement trailers, not the awards themselves.
Not trying to downplay the huge amount of work Geoff does every year. It's just clear to me that people aren't interested in an award show.