If I gave out free hot tubs then there would pretty quickly be a hot tub shortage. The problem is that parking is the leasing of public land for insanely low amounts for private property storage. You are forcing everyone around you to pay for your private property storage because you don't feel like paying for it yourself.
After all, can I pay the council the £100 a year (or whatever tiny amount it is) for an on-street parking spot so I can store my hot tub? Have a little garden? No? Then perhaps we shouldn't let you store your metal box there either unless you're willing to pay market rate for the land.
Keep in mind that any time you say developers must be forced to add room for parking, they were trying to add room for humans and NIMBYs told them places for cars to sleep were more important.