Digital Preservation is an active, long-term commitment; scanning is a time-limited process.
Scanning is a process fixed in time. You scan something once and if you do it correctly you are done and can move on to the next project. Digital preservation is different, because it involves active management over time. If you scan and then forget about the digital file, it may not be usable to future users. You have to look at it as a long term commitment. Think of it in terms of our orphaned data being adopted by a loving digital preservation department somewhere.
If you have someone who needs to be reminded quickly why digital preservation is important, have them read Snawder's blog post.