Because some trains are more expensive than others, or historically have received less income from Eurail/Interrail to cover the large amount of passholders making use of those trains (Spain and France mainly).

Reservations are usually a fraction of the cost of regular 1st class tickets, with some exceptions. So you're not paying for a new ticket, just a premium for specific trains. For instance, while sometimes tickets for the Paris-Barcelona TGV cost €39 (2nd class), when I wanted to book that train a regular ticket costed €229 (in 2nd class!). A passholder reservation just costs €35 (in either 1st and second class).

However, in many countries (such as within the UK, Germany, Czech Republic, Austria & Switzerland...) seat reservations are generally not required.

Because the Global pass only is your ticket and it doesn't give you the right to a seat. The ticket, which gives you the right to travel with the train, and the seat reservation, that gives you the right to a specific seat, are separate.

For trains with mandatory seat reservation you need to make an additional seat reservation in order to travel with the train.

But around 90 % of the train included in the Interrail/Eurail offer have optional or no reservation. For some trains with optional reservation it can be wise to make a reservation during high season. 

If you share your travel plans with the Community you will get advice on which reservations you need and where you best make them.

Or just because some operators (SNCF in particular) see pass holders as cash cows, especially on international trains.

Just as Indian Rail also had special charges in INR-for doing that RES for long-dist trains-also vary with type of train and what 4th rate of 2nd tier or whatever they call it there you want. Since once they started to offer the distinct clients the chance to RES seats/spaces state-run railways have charged for that and this remains till the day. Once there was even a universal price for it-even before this € came.

And again: well over 90% of all trains in the pass scheme are without even chance to RES- but all you 1st time always complaining tourists want is to use these special hi-speed trains.