I write software for a medium-sized software company, and it's hard to quantify how much of the profit the company makes is attributable to my work, since:
- I work alongside many other developers, and
- My software wouldn't make any money if it wasn't being sold by good marketing people, supported by good support people, etc.
So it's hard to know who should get the credit.
If you want a share of profits as a SWE, the standard advice seems to be to start your own company. Then you (and your co-founders/investors) can keep 100% of the profits (minus what you pay in taxes). The downside is that profits are not guaranteed - your net income could be very small or negative, even if you have a great product.
Another way to think of it is that in business, profit is a reward for taking risk. If you're just an employee, you're not risking much by working for a company. The shareholders (investors) of the company, who put up capital that they can lose, are the ones who are entitled to the profits (and can voluntarily share them with the employees if they're generous).