You are correct that it does take time to do ports. However, proportionally speaking the port process is significantly smaller than writing, acting, development (the initial development process for the first platform), post-production and marketing.
Purchases on one platform are largely fueled by the overall marketing machine. The game displayed in advertising is the same and any platform-specific info is relegated to a symbol on the last few frames of a commercial. It doesn't take an entirely new campaign to get that product in front of someone. If there's any marketing going on in today's market with multi-channel marketing being status quo, the people who are going to make a purchase have already decided to do so. The port process is largely automated with most time being spent on performance tweaks.
However, each and every sold copy is meant to subsidize that entire production effort. If I've already contributed to that effort I should not have to pay for subsidizing the entire effort again when all I should be paying for is the effort of making the port, which I have yet to see any publisher do. I think that's the immorality jcb94 was talking about: by forcing people to buy new copies from platform to platform of the same game at full price. If I had the option to supply a serial for one platform and simply pay a "platform fee" for a different platform I would be all over it. But, I will not pay twice for the same effort. That is throwing money away. No business would do it nor should I.
For me, at least, this plays into my mention of it being a service issue. If there was the correct option to pay and that option were reasonably priced I would pursue that option. But, there's not so I'm left to my own means to gain the same effect.