The reason for this is exeedingly simple.
Marx theorised that instead of seeing the people behind transactions, rather we focus solely on the trade itself, the commodities involved. This is a simplification of the theory of commodity fetishism, so to say that commodities are fetishised by focusing on one particular aspect of the product rather than the whole web connecting the products, buyers, sellers, labourers etc.
And actually, developers are merely capital. Their labour time is purchased, just as the computer is purchased, or the loom or silk or bale of cotton. It's simply a transaction which is factored into all the other costs, there's nothing special about labour.
From Wage Labour and Capital:
>Consequently, it appears that the capitalist buys their labour with money, and that for money they sell him their labour. But this is merely an illusion. What they actually sell to the capitalist for money is their labour-power. This labour-power the capitalist buys for a day, a week, a month, etc. And after he has bought it, he uses it up by letting the worker labour during the stipulated time. With the same amount of money with which the capitalist has bought their labour-power (for example, with two shillings) he could have bought a certain amount of sugar or of any other commodity. The two shillings with which he bought 20 pounds of sugar is the price of the 20 pounds of sugar. The two shillings with which he bought 12 hours' use of labour-power, is the price of 12 hours' labour. Labour-power, then, is a commodity, no more, no less so than is the sugar. The first is measured by the clock, the other by the scales.