Because it's costly to build retractable roof stadium. Recently ECB or some official from the Lords also thought of having one and estimated budget was over billion dollar.
Even BCCI don't have enough money to build one, even if they have, they will struggle to sustain it because to generate revenue it has to be multi sports stadium. The Australian one is mostly used for AFL and BBL matches chip in here n there.
Please tell me, which sport other than cricket will use the covered stadium in India?
Does it really cost $1 billion in India? I don't think so. It may cost that much in Europe. And given the popularity of cricket in India, I don't think that it needs to be a multi-sports stadium. One that is solely designed for cricket can work. IMO, if a stadium can be built at around $250 million, with another $50 million in maintenance cost over a period of 30 years, it will give a good ROI. This means that the net cost is around $10 million per year, which is a small fraction of BCCI annual revenues ($2-$3 billion).
While I agree with cost difference argument due to labour cost but IMO you are being conservative with your estimates. if we take this factor into account then cost could be close to half at best, dont forget cricket stadium need big roof in comparison to football etc.
Existing stadium are not suitable for retractable roof so you need completely new stadium so it will also effect the budget. Latest motera stadium in gujrat costed around $100 million so almost same ballpark if you are aiming for top notch facilites.
Let's say price comes down to $400- 500 million but then agreeing on venue would became political drama as BCCI dont control the state boards.
It's just budget and we are not even diving into sustainable model. For example: in India venues are not fixed for international matches because of rotation policy so on average it's around 3-4 matches every year, IPL does help main cities and stadiums but still they struggle to touch 2 digit number in matches. Exception are there but it depends on who is the big dog in BCCI. Jay shah favoured Motera, similarly Srinivasan and Ganguly opted for their respective states.