While there are numerous reasons why a Healer might not be able to throw a heal on someone as soon as they've been resurrected, I'm sure OP is mainly referring to situations where yes a heal is applicable for the person that was just raised at the time. I myself main a healer class and it's normally just a train of thought for me to throw a heal at them /if/ I can after they've been resurrected. Not only that but chances are they'll need a protect too, right? If I have a quick moment at the time I will do that too.
There are too many variables though for why a healer may or may not do the things spoken of above. Sometimes they can't help it due to their limitations but it's not like some aren't just either being very lazy or way too focused on trying to be a DPS (Not saying DPS focus is a bad thing)