Beyond normal consequentialism (as discussed in other answers), there's a game theory angle, where if you aren't trying to model a norm into existence, it's worthwhile to only follow the norm once it's agreed violators will be punished.
 
See paulfchristiano's post on Moral Public Goods, which argues that you will often get into situations where people would be in favor of a norm were that norm enforced, while not being in favor of the behavior the norm calls for when the norm is not enforced.

Everything has an opportunity cost. I'd claim that when impact is very small, it is almost always the case that the opportunity cost is not worthwhile. In general, one can have far more impact by focusing on one or two high-impact actions rather than spending the same aggregate time/effort on lots of little things.

Much more detail is in The Epsilon Fallacy; also see the comments on that post for some significant counterarguments.

(I'm definitely not claiming that the psychological mechanism by which people ignore small-impact actions is to think through all of this rationally. But I do think that people have basically-correct instincts in this regard, at least when political signalling is not involved.)

Your example about cell phones is a prisoner dilemma. The choice to continue using the cell phone has more utility for each individual participant if they are the only person who would stop using it. At the same time it there would be higher utility for everyone, if everyone would choose cooperate in the prisoner dilemma and stop using their cell phone.

Having a government legislate that everyone picks cooperate in a prisoners dilemma is a way to solve the prisoner dilemma.

Even if a person wants to do something about a problem, it's often much more impactful to donate to an effective charity then to change personal behavior.

The recent founders pledge article on climate change that illustrates that principle for climate change. Animal Charity Evaluators might not be the most trustworthy source but when it comes to the numbers I see from EA's the same principle seems true for that area as well.

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Possible explanations:

1) Many impacts are not just small, but effectively zero, or even slightly negative. Spending more effort/resources to do things that APPEAR good but actually don't matter, is a net harm.

2) Some items have threshold or nonlinear impact such that it's near-zero unless everybody (or at least more than are likely) does them. This gets to second-order arguments of "my example won't influence the people who need to change", but the argument does recurse well.

3) The world is, in fact, full of irresponsible people. Unfortunately, it's mostly governed by those same people.

4) Reasons given for something don't always match the actual causality. "It wouldn't matter" is more socially defensible than "I value my comfort over the aggregate effect".

5) Relative rather than absolute measures - "I'm a sucker" vs "the world is slightly better".

6) The https://en.wikipedia.org/wiki/Bystander_effect may not be a real thing, but there is an element of social proof in the idea that if most people are doing something, it's probably OK.

If my action has a zero or infinitesimal positive impact on the relevant problem, while a negative and non-infinitesimal impact on me, cost-benefit analysis concludes I should not do it. I think OP needs to do more work to justify why they think this is not so.

Does this phenomenon have a name?

Laziness, apathy, indifference, lack of self-responsibility, weakness, stupidity, selfishness, herd mentality?

Ultimately the only person's behaviour you can change is your own. Either you chose to do better things or you don't. Lead by example if you care, otherwise you don't care enough to change.