Because if everything to do with Apple technology is proprietary, a consumer is "locked in" to paying Apple for every piece of equipment they buy.

You cannot just got to Amazon and buy a $2.00 charging cable when the one your phone came with craps out, you have to buy the $8.00 one instead, because Apple makes one that is not universal. The "Apple cable" isn't any different or better, it just guarantees revenue for Apple.

Same with interfacing of different devices. Apple makes it so that it is less than seamless to connect your phone, tablet, etc. with non-Apple devices, so that consumers are more likely to expand their inventory of Apple devices and guarantee further revenues.