[jbay808]

One reason you can get away with this sometimes is that ADCs themselves act like a low-pass RC filter. The ADC input is itself a capacitor that gets charged to the voltage that needs to be measured, and it has an appreciable resistance (which can be supplemented with an additional input resistor). Sometimes that's all the low-pass filtering you need to prevent aliasing, and the rest can often be done in the digital domain.

But if the signal you're measuring is very weak (or poorly matched to the ADC range), you probably need some kind of amplifier for it anyway, and in that case you may as well make a filter out of it at the same time to maximize the signal-to-noise ratio.