Because consumers want faster speeds, lower latency, and/or expansion of coverage (to areas that currently don't have fast speeds - or even broadband internet at all).
That all qualifies as investment. If we were talking about something that is a pure commodity utility with pretty much no ongoing technological improvement, like water[0], then investment is less important than ongoing maintenance.
But our Internet is a piece of infrastructure that's constantly evolving, and keeping up with increasing expectations requires investment.
> Are they serving more customers, and is customer satisfaction improving?
Serving more customers almost certainly requires investment. Customer satisfaction probably does too.
Yes, there are other factors which affect those two metrics besides investment, but investment is one that is very easy to measure and is a leading indicator (whereas those two are both lagging - the results are only observed long after the actual work has taken place).
[0] There are plenty of technologies associated with better and/or more environmentally friendly ways to supply and deliver water, but the point is that to the consumer they're all the same. Nobody is clamoring that they want water 2.0, which is wetter than water 1.0.
I agree that dollar amount is probably not the best metric, but it was one of the metrics that companies argued would be reduced as a result of common-carrier regulation. So, I suspect this article is a direct response to that. Despite the likely existence of better metrics, since the companies claimed investment would be hurt, it's useful to actually examine their claim directly.