Since advertising spending is largely zero-sum, additional spending on advertising may shift market share, but it does so without delivering new factual utility or consumer surplus. The fact that there's a pareto-superior equilibrium makes it a symmetric reciprocal externality: Robert H. Frank showed, in "Positional Externalities Cause Large and Preventable Welfare Losses" (2005) that positional arms races are shaped exactly like environmental pollution.

But, even if there's a better equilibrium, isn't loading a website crammed with obtrusive ads still a coasean bargain between two parties? Simon P. Anderson and Stephen Coate studied this question in "Market Provision of Broadcasting: A Welfare Analysis" (2005). They identified unpriced externalities like the common-pool attention tragedy, as has been raised in other comments here, and asymmetric information & hold-up, which increase transaction costs.

Divide advertising spend into two components: Informative specs, and persuasive/obtrusive spend. The margial external cost is the sum of profits foregone by rival businesses, and aggregate viewer nuisance. A pigouvian tax with an exemption threshold for factual spreadsheets and apunitive rate on expenditure above that (or visual intrusion metrics) both prices the cognitive nuisance imposed on readers, and collapses the positional arms race between rival firms; transforming the industry from a high-waste prisoner's dilemma to a low-cost, high-information pareto optimum.

Obviously, you still have to watch out for firms goodharting the spreadsheet metric, but that's both a partially solvable problem, and a much better situation than the current nash equilibrium.