At a basic level, if you're looking at "fundamentals" it's because of this:
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The growth isn't linear. As a company becomes more profitable, it's able to invest into more things and grow in more directions at once-- so you'd expect superlinear growth. If you assume ROI is constant for all profit levels, then you'd expect exponential growth, but it's really hard to maintain that kind of ROI. For small businesses, there are market forces that buffet you, when a company gets as big as Apple it's hard to keep that level of efficiency and to find markets big enough to grow into.
That chart isn't a smooth curve for a lot of macro-economic reasons though too. Recessions, trade wars, pandemics, all affect a company's fortunes. Right now, we're in a recession, but tech is outperforming for two big reasons: they can continue operations because their workers can work from home in a way that other industries can't, and demand for hardware and services from companies like Apple has remained strong over the past few months because people need computers to work from home and they're seeking entertainment in their quarantine.
What happens when the rush of work from home hardware orders fades and people's loss of income starts to really impact their buying decisions is a question for the future.
The other reason why they're doing well is because they're doing well-- there's a lot of savings sloshing around looking for a return, and it seems to be pooling in businesses that are performing disproportionately well versus the market.
I think this is different than the "fundamentals" argument above, this is investors chasing returns and whether the fundamentals are up or down, the choice is between Apple and some other stock (versus Apple or nothing). Non-stock investments net zero return right now-- bonds, cash, etc. Again, it'll be interesting to learn if a change in peoples incomes over time and a roll off in sales drives money out of Apple disproportionately to the market.