Because Northanger Abbey is very funny. It is A THOUSAND TIMES FUNNIER if you have just read the various gothic novels she is spoofing, but even if you haven’t it’s still at least funny-adjacent.
So. Catherine reads many a gothic novel. Catherine herself is an unlikely heroine, because she is neither saintly nor lovely nor prone to faints. Her mother still lives, her constitution is healthy, and when she is finally ‘abducted’ to Northanger Abbey it is less of an abduction and more of a cordial invitation. From a handsome man (and his sister, let’s not be inappropriate).
Catherine arrives at the abbey and is disappointed to find that the windows are actual windows and not slatty casements, and that the long-shut-up room of the late Mrs Tilney is a bright, airy chamber. Alas, right? No mysteries here.
And it is very like an Ann Radcliffe novel where something verrrrrrrry spooooooky will turn out to have a realistic explanation, except that in Radcliffe the mysteriously-appearing ghost will turn out to be a corrupt monk who sneaks in through a hidden door when you aren’t looking, whereas in the Austen said ghost will be, like, old linens in a pile. And where the former makes you pffffft in disgust, the latter will sometimes make you HAR out loud.
And it was helpful, even having just stewed in The Mysteries of Udolpho myself, to be reading the Oxford World Classics edition because gothicky language and Austeny language are too close to each other and too far from me for me to segmentize them, so it was helpful to have ye olde asterix to be all, Austen is parodying typical gothic description here. If you are not steeped in gothic tropes, aforementioned asterices will, I think, augment the funny.