Cost isn't the whole reason, either. If you think about it, you have to LOOK at gauges, and I'll bet most of us have gone for quite a bit of time without even thinking of checking engine instruments. They never seem to change much, do they, so why worry about them?
With engine monitors like the Rocky Mountain, or Grand Rapids EIS, and others, flying is much safer because all of these types of monitors have programmable alarms which are a lot more useful than a passive gauge just sitting waiting to be looked at.
Even if you want to use all standard engine gauges, it would still be a wise idea to install active alarm indicators for at least the important basics like voltage (hi AND lo) and the liquid pressures and temps.
But with EIS monitors, you can set alarm limits for anything which can be quite useful.