Steem has a built in inflation, right now that inflation if close to 350% by the one year mark that will be down to 100% inflation. The inflation is designed to distribute funds to new users, and prevent anyone from having too much power. This is driving the price down, but very slowly. In reality the marketcap of steem is going up.
The 2 year payout is part of what makes steem a good investment, it makes it very hard for anyone to crash the system, especially in panic. People bought bitcoin even when it was waaay down from $20 at $12. Steem as a platform is continuing to grow, it's continuing to get media attention, and more people are contributing than ever.
I can't say why any indivdual is investing in steem but i imagine people are investing in steem because they believe that it will be a good investment in the long run. Once the inflation slows down, the supply will go down, and the platform will likely be used by more users than ever. There is money to be made on steem if you contribute to it, which is how the platform is designed which encourages people to get involved. Other's might just see that it has a great potential and are investing in it because they believe the price will go up over time, even if it is going down right now.