Airport shops are a captive audience rip off, relying on people still thinking they get “duty free” on everything and therefore must be cheap.
For a start only cigs and booze are duty free now anyway and only when travelling outside the EU (post-Brexit though this likely changes).
And then they may advertise prices before VAT, but the price is the same as the price with VAT. When asked for your boarding pass in the shop they determine the destination and if you are travelling within the EU, VAT is charged as per the law, and outside the EU, they charge the same price and pocket the difference.
http://www.independent.co.uk/incoming/airport-shop-vat-savings-what-you-need-to-know-about-duty-free-scam-10451207.html
Besides that, they’re big name retailers with high prices compared to budget high street and online shops.
Not to mention also that often the same products may be cheaper still in the destination you are flying to (so long as it’s not an airport or tourist trap shop).