It's not that I want to read all the decks first and then watch all the pitches, it's that I want to look at the decks in order to decide which pitches to watch. S19 demo day had 197 companies. I don't know how long the pitches were (I didn't go) but if each one was 5 minutes that is 16 hours worth of pitches without a break. I'm not capable of maintaining focus at a presentation for much more than an hour or two at a time. If my attention wavers at the wrong moment during a pitch I might miss something important.

If I can look at a deck, then I can do two things that I can't do during a pitch. First, I can close my eyes and take a deep breath for a minute without missing anything. And second, I can see right away that, for example, this company is in a sector that I am not focused on (because I don't know anything about it) and so I am not going to invest in it even if it looks like a great opportunity. I am an individual investor with a very narrow focus (I look for companies in particular technical niches) so I can eliminate 80-90% of opportunities just by knowing the market segment they're in. A company might be well positioned to revolutionize (say) fashion design, but I'm not going to invest in it because I know absolutely nothing about fashion design, and I don't invest in things I don't understand. With a deck I can figure this out an order of magnitude faster than if I have to sit through a whole pitch.