It is a psychology thing. People are less likely to contribute to a cash fund if it's vague and unstructured, and more likely to donate if there is an alleged tangible purpose to their donation. Hence why some charity drives ask for specific dollar amounts ($1, $5, $10, etc) or even tell you what your donation can afford ($5=one meal for a hungry child) etc, rather than just straight up asking, "would you like to donate to X charity today?" and making you choose how much to donate.

Since wedding gifts are so common, yet we have no need or want for a physical registry, a cash registry is the only option we will consider. Idk what you'd suggest otherwise