People love to get mad at a scapegoat rather than a root cause. So you blame the scalper rather than realizing that the concert you bought the ticket for set their prices way too low - perhaps because selling at the "true" price (somewhere between the sticker value and the scalped value) - would generate negative goodwill, or it would give the perception that an event (say Coachella as the classic example) is only "for rich people". The irony is of course that selling at an artificially low price doesn't suddenly make it not for rich people, it just adds a small random chance that you'll be able to convert your own time sitting there refreshing the website across many browsers into a cheaper-than-it-should-be ticket. But functionally the vast majority of people end up buying from scalpers or at least at a slightly above sticker price.
There's a related discussion about so-called "price gouging". IMO it's the classic scalper argument but just in a different form. Price gouging laws are not just immoral but are actively ineffective at their stated purpose; they just lead to the hypothetical price-gouge transaction never occurring. If you drive across state lines to bring HAM radios or water filters or what-have-you during hurricane katrina, and try to sell at above market rate, you'll be tossed in jail and your goods will be seized by the police, rather than making it out to the people who actually need them and are willing to pay 2x or 3x the usual price.
TL;DR: We've all got these twisted beliefs about how pricing works or the law of supply and demand, an illusion supported by the fact that our respective societies are so technologically/economically advanced that we can have massive supermarkets where you buy commoditized items that you can get at reasonably similar prices anywhere. Whereas if you go back in history to a pre-industrial time, a form of pseudo-bartering was the default and there was not this idea of a commodity, but rather every transaction, every buyer:seller relationship etc was a unique and non-commoditized interaction and thus it wouldn't seem weird to have different prices for different people or different prices at different points in time, etc.