You've unwittingly head the nail on the head with your comment about being 'friends'.
A workplace is a professional environment, which requires fair treatment of everyone. Friendship doesn't come into it. You cannot have a clique organising things for each other in the office because 'they' are friends, and ignoring everyone else in the wider team. Which IME is often the case!
I have no problem with centrally managed celebrations, in former teams we had a calendar of significant events with the boss's PA organising gifts/cards. Paid for with his/her corporate credit card.
The issue with self-organising however is that only the popular ones, or those who have 'event happy' friends in the team, get celebrated. Quieter ones often get left out. It's extremely unfair especially if the latter are also expected to contribute, and get nothing in return!
Furthermore, if you read the OP carefully, she says 'we' don't organise baby showers for dads. Of course, it could mean the royal 'we', i.e. society. Fair enough to only organise for the birthing parent. But if one is organised for a female solely because of her sex, then what's the reason for leaving dads out? It's sexist and discriminatory.
Also , requiring people to fork out for gifts is unfair. Good for you that you earn enough, but others may be counting pennies. It's really not inclusive. A signed card - fair enough. But the 'shower' in the name refers to a shower of gifts, which is clearly inappropriate.
Have a standard policy, paid for by the company /with a fair, low effort distribution system. Or ban all of it is my take. Nothing to stop people organising privately, outside the office if they want but there should be no pressure on colleagues to attend or contribute.