(A) MN hates weddings, both as the couple getting married and as an invitee/guest. According to received MN wisdom anything more than getting married in jeans (why always jeans) down the registry office with 2 witnesses ideally strangers pulled in from the street is hideously extravagant and showy, a total waste of money, and an enormous inconvenience to your guests who will inevitably be hungry, bored and thousands of pounds out of pocket thanks to your selfishness in inviting them. How dare you then indicate that this is a social occasion on which presents are typically given and be clear about what you might like or have a use for? Are you a monster raised in a ditch? 
[B] MN hates occasions on which gifts are typically exchanged (cf adult birthdays, anniversaries, Valentines day, mothers day etc) and thinks any adult that likes or, gasp, expects to receive a present, ever, is shockingly 'grabby'. No form of gift giving whether it's home-made crafts, 'experiences', wishlists is acceptable to MN and cash-giving is the lowest of the low...
[C] The poems are always twee and shite. No beef here, that's 100% true. If someone can post an example of a cash-poem that isn't this I will doff my cap to you in awe and wonder.
[A] and [B] are illogical to me, and in my experience of the 'real world' the vast majority of brides/grooms would prefer cash as a gift and the vast majority of guests happily give it too, within their means. In fact many, many cultures around the world have always had a tradition of cash giving rather than physical presents (often with some fairly clear and well-understood social expectations of how much you should give according to your relationship with the couple) and it's way easier to navigate than our rather arcane and coy rules about gift lifts and wishing wells and poems and having to to ask a million times 'no really, what do you want?' and be told 'oh just your presence, really', when everyone knows it's rude to turn up to a wedding empty handed so you have to take a stab at whether £50 is stingy or too much for a couple where you once worked with the bride and are invited as evening only guests and everyone is mutually shamed and embarrassed about the whole exchange...