For example any instrument costs much more than that, and the same time could have gone into developing the software and building the instrument. Or heck, any handbag or fashion item can cost a ton of money, and people are ready to buy them, but when software actually costs something close to real value produced by that software, people get pissed these days.
I blame the iOS/Android markets that have created a biased and untruthful image for the costs of developing software. You can't expect to make a living from developing software that costs 1.99/2.99/9.99 unless you sell a ton of those, and most never do, but this market has effectively changed the mindset for normal consumers to get things either free or for a price of three cups of coffee.
2. It's not $750, there are multiple editions available, the more expensive ones include more plugins, instruments, loops, etc.
3. Cloud-based startups are nice until they reach the end of their wonderful journey, sell out and you're out of a tool.
Soundtrap is charging $15 because that's what it's worth. Although if the cloud is mandatory it's worth zero in my opinion.