That's because it's a misunderstanding to describe an RO unit as a mere filter. Filtration (which is not alone what RO does) will not soften water.
The major cost for small RO isn't the power, it's the filters. RO units will be knackered (expensively and rapidly) if you feed them poor quality water. So they depend on built-in pre-filters, which are better than anything specified for house use, and themselves aren't cheap.
There's also a risk with RO units that the resultant water is somewhat acidic, and so you can have a metal contamination problem owing to pickup from the downstream pipework. It's not a good idea to follow an RO unit with long runs of copper.