I always wonder why people move into a dump run by a crook, let alone stay for years when nothing gets done. And don't tell me there's nothing else and that all rental properties are shitholes. Keep paying crooks, they keep thieving.
There's a massive shortage of rental properties, especially in London. I live in Zone 3, not a popular or particularly desirable area, and a 3-bed rental property will set you back around £1,700pcm. If you want three largish (as in not boxroom) bedrooms, or a garden, or parking, or be close to a decent school or station, or want somewhere that has been refurbed within the last few years, or has proper gas central heating and decent windows/insulation - add another hundred or so pounds.
If you look on Rightmove, it looks like there are loads of lovely places, but ring up the agents and you'll be told it's 'just gone.' It probably hasn't, they probably let it out months or even years ago but relisted it because it looks great and will get people calling up, who they can put on their books and then show the crap properties to. My last rental was beautiful and popped up on Rightmove a couple of times (I kept the email alerts on) but definitely wasn't up for let - when we called up posing as a potentially interested tenant we were obviously spun the 'it's just gone' line and offered other viewings. This was a reputable chain of agents as well.
Once you've found somewhere that ticks a couple of your boxes, you then need the available date to tally with the date you're moving out, else you'll either be paying out double rent and deposit, or you'll be homeless. So the perfect place might come up but it's either available right now or in three months' time, but you need somewhere in 4 weeks.
So you finally find something you like and manage to be the first one to offer market rent and everything tallies up date wise, you move in and guess what? The oven is on the blink and it takes three hours to cook a quiche, and the ignition switch on the hob doesn't light, and the back door doesn't lock, and the seal on the freezer is cracked so it frosts up really quickly, and the shower is basically a dribble, and the hot tap on the bath dispenses cold water, and the cold tap hot. (All of these have happened to me in my last two rentals). If it was your own home you'd fix everything in a couple of weeks/months, but as a tenant you seemingly have to pick your battles - so you decide which are the most important things to fix (oven and back door, in my case) and which you can maybe solve yourself without causing damage (freezer) and which you can probably live with (hob, taps and shower) so as not to vex your brand new landlord.
That's basically how and why.