Because this creates a big competition, which is very good and very needed for the network stability and quality. Because only 101 can become forging delegate, those 12k LSK create a big incentive for delegates to provide top notch quality (powerful servers, good protection, well-thought monitoring tools, big amount of invested time). If a delegate would only earn a few bucks per month, nobody would care about becoming delegate and the network would be much more unstable and vulnerable.
Additionally, the 12k LSK allows the delegates to spend parts of it for pushing app development and supporting the complete Lisk ecosystem.