When time is money, you don't want to wait on a computer - and the tasks are becoming much harder (4K video vs HD video vs SD video for example).
That's true up to a point. If I could shave 5 minutes off a 1 hour render time (that's an 8% increase in productivity), is it worth $500, $1,000, $2,500, $5,000? There's a point of diminishing returns for any business. It's an asymptotic curve and at some point the outlay doesn't justify the return.
Case in point
say I have a worker who I pay $100,000 per year (who works 2,000 hours a year - 8hrs per day * 250 days). That means the person makes $50/hr. Now let's say I buy him a rig that shaves 5 minutes off his render time and he renders this twice daily. I've saved $8.33 each day. Now let's say he does this every day of the year (250*$8.33) and now I've saved $2,083.
Now, let's say that in order to save the worker this 10 minutes daily, I have to go from the hex core to 8 core rig and upgrade to the D700s and give him a RAM boost from 16GB to 32GB. All told I need to spend an additional $4k (going from 6 to 8 cores is like $1k and a pair of D700s are going to be around $3000 with a few dollars thrown in for the RAM).
All of a sudden the break-even point is at about 2 years and then it becomes cheaper to just buy the person a new computer rather than spend it on the front end.
Here's another point to consider. Rather than spending the money on the rig, I could invest the money and earn an 8% return. Compounded over two years (instead of spending on the upgraded rig) and I've earned $665.60 on my original $4k investment. Still using the $8.33 per day savings, by not buying the person the upgraded rig and investing the money instead my opportunity cost has increased by ~80 days (665.60/$8.33).
Again, I won't deny that there's money to be saved by buying a faster, beefier rig, but at some point the costs outweigh the gains.