Sector ETFs are already pretty inexpensive on an expense ratio basis. Vanguard's sector ETFs for example have expense ratios of 0.14%, which compares with an expense ratio of 0.05% for the cheapest S&P500 ETF. A bank wouldn't be able to do it any cheaper, realistically. Someone could offer ETFs that exclude particular sectors, but it just hasn't been done, and I still don't think it would be cheaper because of economies of scale for the funds that currently have the most capital.

You do have to have a certain amount of capital to successfully diversify using ETFs, obviously, but the bank doesn't really care about you either if you aren't investing at least a few thousand.