Separate voltage regulators for different digital ICs
High-speed chips have high-speed current draw. Thus, putting a long line between supply and consumer of current is an undesirable thing, because it introduces parasitic impedance. That's like you soldering an inductor in series with your supply!
Also, it's often way easier (and cheaper) to design a board with one (or maybe two) power rails going everywhere, often even on their own plane, and then convert down to lower voltage where that is needed,  than to distribute many different voltage rails with a low impedance.

Is it better to do this instead of using one larger 1.8v regulator for several ICs, which seems for efficient?

There's nothing more efficient about having a single linear regulator instead of many. It might be cheaper in terms of costs of regulators, but again, you'll pay for that by massively complicated board design.