Relationships are a two-way street. If one person is routinely putting forth all the effort, expense and time into celebrating the other side of that relationship whilst the other party just takes all that effort and gives nothing back-it's no wonder people become resentful.
It's not about monetary value, and I resent the notion that to be bothered about the imbalance is to be "selfish and materialistic."
No, it's not. Single (and sometimes couples, too), childfree/childless people are often put on the back burner in their friend/family groups once everyone starts getting married and having children. Yet, being expected to routinely step up and be present-both physically and financially-at weddings with gifts, baby showers, their kids birthdays, Christmas, Easter...
All the while, any milestone in that person's life is ignored. Bought a new house? Nothing. Got an amazing promotion? Ignored. Birthday? Ignored. Christmas? Make sure you buy for the kids but we're not going to buy for you or gift something "from the kids."
Being routinely expected to show up as a visible presence in their life to celebrate their milestones whilst your own are ignored and dismissed as less than because you neglected to or were unable to have a child breeds resentment. This is common sense, and often, something a simple card or a token gift would cure.
It all comes down to effort.