This was normalized on the web long before these companies started - they're just the best to do it (though Google did acquire one of the earliest players in DoubleClick).

The term "conversion pixel" is a misnomer these days for a Javascript snippet or API call - the concept predates JS and used to refer to a 1x1 transparent GIF.

Anyways, that generation of tech is being pushed out due to browser privacy enhancements and will be replaced with privacy-protecting browser and server APIs that introduce noise to the data, such that you can still measure the directional/relative impact of advertising without being able to tell exactly who converted.

There are a few competing industry proposals involving differential privacy/cryptography/blockchain/fixed entropy that are being publicly discussed here: https://github.com/w3c/web-advertising