The main reason? Licensed themes continue to sell. They aren't remaking Castle and Space because those lines are dead. Also, you'll find that generally speaking Lego reuses designs every few years, or brings back lines similar to other ones (Castle > Knights Kingdom > Nexo Knights), (Life on Mars > Mars Mission)
So there are two things you're missing here. For starters, a ~200 piece increase is not a small increase. An 8 year time gap is not just "newer" that's almost a decade. Therefore you have to account for inflation when you complain about prices:
"According to the Bureau of Labor Statistics consumer price index, prices in 2016 are 11.47% higher than prices in 2008. The dollar experienced an average inflation rate of 1.37% per year during this period.
In other words, $400 in 2008 is equivalent in purchasing power to $445.90 in 2016, a difference of $45.90 over 8 years."
Source From Here
What that basically means is that your 100 Dollar increase that you see is actually only a $54.10 increase when accounting for inflation, since the cost of that set in 2016 would reasonably have been 445-450 dollars, and that therefore a 200 piece increase in the set costing an additional 50 dollars, while costly, isn't actually all that unreasonable for an exclusive set like the Death Star.
As it stands, personal feelings aside, You can't call the Death Star (a re-release from almost a decade later) and Tahu (Who had 2 versions in three years with almost the exact same build, and then another version years later with the same build that had been used for Four Years by that point) entirely different. One is a limited edition release meant to appeal to the new generation just getting into Star Wars (and the older ones who couldn't afford to drop 4-500 on a lego set back then but could now) and the other is a copy paste that went on for years.
Literally everything you're complaining about in terms of sets being reused is basically just Bionicle. I hate to say it, but it's true. Bionicle was not exactly the best for always having new things, even if the design and overall look were different.
Point is you occasionally have some valid points against the Licensed themes you hate so much but this is not one of those times.