Why can't Youtube ads be blocked?

If you are on Android use the New Pipe app. No ads.

I recommend you use a yt downloader its hard to find one without the ads being broken on next dns so here u go : https://youtube-video.download/en1/

is there a way to disable ads in streaming services like tubi.tv or plex on android streaming boxes?

I want to add to what Mohit said (share video to AdGuard and YouTube Vanced).

If you use webpage in good browsers instead of useless apps you not only can block ads but you also do something for your privacy. Some services like Twitter and Instagram and even this website (help.nextdns.io not my.nextdns.io) have https://en.wikipedia.org/wiki/Progressive_web_application PWA . You will have icon and it will look like an app but is running inside the browser. Not all websites have this, last time I used, Facebook, for example, did not have it. But it was OK in mobile web browser. You can even use messenger if you change the mobile browser to desktop.

As for streaming without ads, the best thing I know is https://wwww.webvideocaster.com Web Video Caster. Once you get to know how to use it's amazing. I can cast YouTube for example on my TV without ads.

In addition to NextDNS, I use AdGuard (Safari version, not Home) and never see any ads on YouTube.

Pure Tuber >App

YouTube does not implement it's ads via separate domains, it serves them on the primary domains, so blocking said domains would be blocking the service itself.

Computer:

1) Use FreeTube (Recommended to use with Invidious API, proxying through Invidious instance is optional but reduces chance of blocking, although the cost is slower or poor video loading depending on your location)

2) In browser: Use YouTube with uBlock Origin in any of the supported browsers. (aka any browser that allows the uBlock Origin extension to be installed)

3) In browser: Use Enhancer for YouTube in any of the supported browsers (Recommended to use with uBlock Origin at the same time, Enhancer has alot of customizations that users may like, uBlock would handle the adblocking part, although Enhancer can do it as well, your choice)

4) In browser: Use Piped or Invidious (inconvenient and sometimes buggy)

Android:

1) Use NewPipe, NewPipe x SponsorBlock, LibreTube or SkyTube (These apps are known to be quite buggy however)

2) Use YouTube Vanced for as long it's still supported (You'll need to use web.archive.org in order to get the download link for Vanced Manager as the official website removed it late March due to the takedown pressure by Google)

3) In browser: Use YouTube in a Firefox based browser with uBlock Origin (Firefox for Android, Fennec F-Droid, Iceraven or Mull; inconvenient and sometimes buggy)

4) In browser: Use Piped or Invidious (inconvenient and sometimes buggy)

iOS:

1) There are no app-based methods other than purchasing YouTube Premium, there is also no uBlock Origin in iOS.

2) Use AdGuard for iOS/AdGuard Pro coupled with YouTube inside Safari (restricts you to only Safari and requires you to take manual steps everytime you load/reload the website, deemed a very annoying method that forces you to use a browser people dislike, but a method nontheless)

2) In browser: Use Piped or Invidious (inconvenient and sometimes buggy)