You're patient. Some aren't. If you farm your 4 shards and stop you have 4 shards. If you do the same tomorrow you have 8. BUT, if you spend 200 crystals in addition to the 50 used for farming you can have 8 today. That's the instant gratification they're targeting. The math is over simplified for the purpose of the example so please don't point out how it would be cheaper to buy a node and energy refresh and farm 4 more :)
I agree though. I'd be more inclined to spend crystals on shards of the cost were the same as farming then.
For gear they're targeting time saved. That 1400 crystal piece might take 7-10 days to farm. If it has 1 rare 50 salvage requirement. If it has 2 it may take longer. Why wait 7+ days when you can have it now? This is where they get me. I'll my crystals and buy the full pieces once a week or so.
The final piece your missing is opportunity cost. If you don't buy that full gear piece you miss out on the opportunity to farm something else for the next 7+ days.
Prices are high, but when you can get 7500ish free crystals a week it isn't too bad.