When you have 1 BTC, you have 1/21m of the total supply. That's your share of the pie and if you do nothing, it will remain constant forever.
Everything else changes. Prices of things change in relation to the prices of other things. The price of BTC in USD or whatever shitcoin you fancy. The amount of beef 1 BTC can buy. The number of eggs 1 BTC can buy. And the number of eggs a pound of beef can buy. Out the window go the baskets of goods used in CPI indices...
Economic growth is variable too. BTC's market share changes too and it's hard to make any predictions.
The only thing that can be constant is the share of the pie of a money with a fixed supply.
In this unpredictable world, it's strangely comforting to have a money that at least offers some certainty. Even if that certainty doesn't tell you how much beef or how many eggs you'll be able to buy tomorrow - which is impossible under any money anyway.
It doesn't make sense to measure one's wealth in how much beef one can afford, because one day we may discover a new way of growing beef, which may make it as abundant and cheap as air, and people will trade tons of it for an egg just to have some variety.
It looks like the only sensible way to measure one's wealth is the percentage share of the pie represented by a fixed-supply currency. The same share will likely buy more stuff in the future simply because - due to economic growth - there will be more stuff out there. The pie may grow, but at least you can be sure x% of it is yours, forever.
And this feature will be key to Bitcoin's success if it succeeds.