Because high-margin goods aren't spare change. Look at the razor:
I can sell you the razor at cost-plus for $20 on a $19 manufacturing cost. I can sell you each individual razor cartridge at 90% profit for $1. At the end of the year I've sold, say, 5 razors and 50 blades, for a total of $150.
On the one hand, the razors made up the bulk of my gross revenue ($100/$150).
On the other hand, looking at the actual profit I've generated, $45 came from blades and $5 came from razors - that is, 90% of my profit came from those "pennies" that the razors brought in.
If you can strong-arm people into buying high-margin goods, that's a strong bonus to your bottom line, even if the absolute price isn't that high.
Let's plug a fridge into those numbers.
A nice new GE fridge can be had for something in the vicinity of $800 (sears website is showing me stuff $600-$1000, so let's just split the difference). Amazon has GW RPWFE filters up at $50 a pop.
You buy a fridge, what, every ten years? Every twenty? Those filters "should" be replaced every quarter? Let's be generous and say every half a year.
Those filters generate the same profit as the fridge at 16 years out, at the same profit margins. At asymmetric profit margins (say, 30% vs. 5%) they've produced profits equal to that of the fridge in a handful of years.
These aren't pennies.