Makes sense when you consider most towns have a plethora of supermarkets, and they all sell near enough the same thing.
As a family we spend about £80 a week online with Tesco, £10 a few times a week at the local Co-op and £30 at the local butcher twice a month. The only one I'm loyal to is the butcher - a specialist who offers a real point of difference - better quality, better prices and a better service. We've always shopped at Tesco (Clubcard vouchers do come in handy) and the Co-op is on the school run. Pure convenience in how we shop.
I write this because both Sainsbury's and Tesco have new ad campaigns out at the moment, with spending that must run into many millions. Personally, no matter what any of the big shops say in terms of campaign messaging trying to get me to switch, the likelihood of us doing so is zero. Like the vast majority, we shop where we shop because of convenience not advertising.
So rather than spend millions chasing potential customers who are unlikely to switch 'allegiance', how else could they spend their ad millions to get a more realistic ROI? Encouraging me to spend more in store/when I log on (incentives, discounts etc)? Making the stores more inviting? Improving loyalty schemes? The list goes on...
Supermarkets will always gain new customers but they will do so as a consequence of convenience rather than the effect of successful advertising.