Eh, I wouldn't call it an obsession. The simple fact is Walcott is the only player we have (hopefully now not the only one with Pérez, but we shall see) that has pace and makes runs in behind the defence. There tend to be more chances to be had when he's on the pitch, though last season he seemed to lose his way and try to play like an Alexis type player when Alexis was injured and he was on the left, and his pace seemed less, and the chances that he created with his movement he was putting away at an alarmingly low rate.
Wenger's belief in the use of Walcott is absolutely understandable given that he offers something much different from Campbell, who is just meh, or from Ox, who has 'potential' but is either a very, very bad version of Campbell or a more athletic and better one depending on if he ever can play with something of a brain. Given this it's hard for me to understand why people can't figure it out, and why you must resort to calling it an 'obsession', especially because if it was he wouldn't've sat him on the bench for long stretches of time as he did in 14-15 when he came back (constantly preferring Welbeck) or last spring after his long run of poor form.
Wenger's obsession with Ramsey is far weirder and more perplexing, and frustrating. And Ox is an obvious second place. His treatment/use of Walcott is coherent and easy to figure out, tbh...the only thing I can really criticise in it is you'd hope he could've just gone out and bought a better player that offers similar qualities (Lacazette, maybe), but still, this critique comes nothing close to the strength of the critiques we could launch at him for Ramsey and Ox.
It's much of muchness really. Either go with the sure and steady mediocrity of Campbell or go with the upside. @arsenescoatmaker has put it pretty cogently I think. No doubt Theo on his current form offers us more than Campbell. Let's hope it continues and hopefully even betters, to 12-13 levels for example.