As to "why couldn't any major bank or credit card company implement this and blow them completely out of the water", why not ask instead why they haven't successfully done so? They've certainly tried and failed, and for the most part seems like they are turning to investments/acquisitions of companies in this space since they can't build something successful on their own.
I'm sure they'll love it right up until it gets scrapped. Businesses can't run on love alone. Startups need growth to survive, as soon as this product gets big enough to interest the banks or credit card companies they will make their own and it will be better and cheaper since they can eliminate the middleman.
Maybe they're positioning themselves for an acquisition, but anyone who bought them would just be buying the idea of investing change. The app itself would be killed as its functionality would be combined into the buyers existing app ecosystem, and the backend that takes the change and makes the investments would be largely unnecessary as the banks already have the infrastructure to nickel and dime people.
The context of this article was BlackRock trying to get younger customers. To accomplish this, they invested in a startup whose target audience is apparently people who want to invest, but don't really want to put in the effort. To me that sounds like a questionable investment.
You could argue that they're providing the change investment thing as a loss-leader to try to get people interested in investing, but something like RobinHood already gives inexperienced traders the tools with which to bankrupt themselves.
How much can a customer who adds a few dollars a month to his account be worth anything to an asset manager? Won't it take years before even the user acquisition cost is recouped?
I'm not sure they have tbh, not because they couldn't but because it's not high priority even without regulatory and institutional hurdles to cross. Moving additional pennies around doesn't radically alter the fees banks and credit card providers already get from customers they already market savings products to; a neat little app to start building up savings is an opportunity for investment providers that don't do retail services to gain consumer attention and potentially bigger deposits in the long run though though.