This is complicated because of how ad networks work and all the layers involved.
We have somewhat of a standard called RTB (real-time bidding) where an exchange will see each impression and run an auction in 100ms for each ad slot and then take the winning bid and render it on the page. That was easy and efficient when it was just an image. Now ad formats have become more "engaging" and require javascript tags to run (like those expanding ads). So now the exchange tag wont just load an image but load more JS tags to render the ad.
Then we have DMPs (data management platforms) that are solely about measuring your audience. Each one is a isolated silo of data and they dont share because they all have their own algorithms and backend connections with offline data sources to provide value. Most publishers and networks however have to work with all of them since ad buyers want access to everything or they wont spend.
Then we have the new 3rd layer of vendors who do ad fraud and viewability (measuring whether an ad is actually visible in the browser window) which is yet more tags to make sure the original ads are actually rendered and worth something. Then there are some ad networks that don't work through RTB exchanges that need their own tags on the page.
An additional tag isn't the problem, it's the fact that these networks are designed to just offload as much as possible to yet more providers which means a single 5kb tag can balloon into dozens of requests which all load their own crappy frameworks to do the same thing. 10 small tags on a page are fine. 10 small tags that load 25 tags each is what's killing these sites.
Disclaimer: I've built ad networks and run my own now.
EDIT: I wanted to add that part of what I'm pushing for in the industry is more regulation not only with business practices and data but also technical certification. Something to prove that an ad network is actually designed well and won't be a burden on sites/users. Then buyers can just trust that certification without asking for all these 3rd party vendor verification tags that slow down sites and increase cost. Any ideas for making this happen would be appreciated.