Stock and bond indexes, or stock and bond funds, or individual stocks and bonds don't show linear growth, or exponential growth. Depending on the time period they can show growth or decay. Those types of change in a specific time period can appear linear, exponential, logarithmic...
Investments can't show a consistent pattern because the change in prices and value is based not on time, but on sales, estimates sales, perception of hotness of those sales. It can also be based on rumors that can drive the price up or down. Just look at how the markets react to the guess on how a vote will go. Those prices of individual stocks can also be moved by market and industry beyond their control.
Look at your chart, it is easy to find periods of up to 10 years where the growth was zero or even negative. Of course your chart ignores that most people don't buy all their investments on one day and then let it ride for a few decades. My retirement fund started with a initial 401(k) contribution of ~$100 that was matched by ~$50 from my employer.