Because DigitalOcean fought, and possibly paid, for their blocks for you. I don't do it any more but even in 2010-2011 getting blocks from ARIN took real work (ask for /18, get /19, lots of justification and explanation, etc).

Now everybody's pretty much out. So I don't know DO's blocks, but very likely if they're running out they're having to buy more. Depending on how they cut it up a /16 -- I know they have at least one /16, because it came up here yesterday -- can serve at least 60,000 droplets or so.