Let's say that bitcoins cost $600. At $600 there are 0 people selling, at $610 there at 10 people selling, at $620 there are 100 people selling and I assume you got the pattern (the graph below is the actual current version of this from bitcoinity). Instead of buying bitcoins and watching the price fluctuate in response to a 30k buy, these large companies are better off paying a flat rate premium to acquire all those bitcoins without influencing the market too much or overpaying (because by the time they get to their last bitcoin, it would cost a fortune).
![](https://ip.bitcointalk.org/?u=https%3A%2F%2Fi.imgur.com%2Fy83epTy.png&t=663&c=olYwDzT2nl-oCQ)
Also, if you've noticed the logarithmic curve flattening out, I would like to remind you that that means for every new bitcoin, the price will increase a lot more than in the beginning of the graph, but if there is some meteoric rise then there will be more people willing to sell which would correct the flattening out.