Surplus value is the excess of generated value produced by the labor of workers over the wages they are paid.

For example, if I create a software app that generates 3 million dollars and get paid $20/hr for the work over 100 hours, that is only $2000 wages. ~2.9M, minus constant capital for software/hardware, variable for advertising spots and promotion, and still a lot of value driven straight into the capitalist's pocket.

The same does not apply to the machine, which is a large upfront cost and the value generated is consistent, meaning you cannot squeeze it for any more than it is able to do (unlike a worker). In reality, the machine can pay itself back and eventually generate value, but the labor is a better, more stable source, and one that the capitalist often chooses over the machine.