Because they’ll use against you what they have found out.

Advertising networks want to get you to buy things you don’t need, or at places where you probably wouldn’t want (higher price, garbage warranty process).
They are also used for deception campaigns*, which is made more efficient by showing you those ads that may have a better chance to catch your attention.

* an example is false political propaganda, which was youtube spreading in unskippable ads in my small country, before and while elections were going. It was a great success to them. multiple local channels were trying to block these with minimal success (channels have tools to filter ads their guess see), as somehow blocking by category or by uploader channel did not work, only blocking every single such video, which is ineffective because new ads from those scumbags still get shown.

Insurance companies may also change your rates when they find out something about you from data brokers, which includes diseases you don’t even know you have.
Media platforms (social media, video sites like youtube, movie sites heavily using recommendations) know your rough (or finer) worldview, your interest, what makes you click and such, and they’ll first put you in an echo chamber where they form your opinions by showing you news and other media that you want to see (which is not necessarily true, and can also miss the bigger picture, but you’ll most likely accept it with the least questioning), and then they also form what you see by their agendas: they know what gets your interest, they can use that to show content that makes you stay for longer, and they can also show content that spreads information that is in their interest and about which they know you’ll still read it or at least think about it.

Data points get connected. Ads are served. For some people, that’s enough of a privacy violation. The next step is selling your information to insurance companies.

Me too, but I have at least a dozen of firefox windows and hundreds of tabs combined, all of which are restored on starting it (because I want it this way, I could also just disable restoring them if I wanted).

Also don’t forget that there are addons that make the browser waiting before continuing to start up, and with a good reason.
One such is uBlock Origin, which needs some time to load it’s filters into memory, especially if you have enabled more than the defaults. If it wouldn’t do that, it couldn’t do filtering for the tabs that get loaded right at startup, and that would be quite bad.
On chrome, addons can’t make the browser to wait with the first network requests (but also can’t do efficient filtering anymore thanks to Manifest v3 changes, brought to you by your favorite advertising company), so chrome will inevitably be able to start up faster, but with a huge cost on your privacy, because uBO and such firewalls can’t do their job properly.