I like spontaneity, but it also can mean that the thing you want to happen doesn't, so it needs to be a balance between that and planning.
Big groups: need to plan so everyone is free to attend
Want to visit a particular restaurant - in my city, if you don't book, you probably won't get in anywhere decent Thurs-Sun without a reservation.
Ditto things like spas, or theatre, gigs, holidays etc - need to be booked in advance or is unlikely to have space for last minute walk ins.
Childcare is a legit reason, but even people without kids, we make other plans with other people or for other things - your floaty last-minute-ness is stressful when I want to see you, but also want to get my errands done and see other people and do my hobbies and and and...
It tends to end up with the organised person waiting around for the "spontaneous" one to be ready, and so it often comes across as selfish and thinking that your time and convenience is more important than other people's.
It's nice to get a last minute call of "oh, it's a lovely day, let's do X", but you need to be ok with me saying "No, I already have plans" most of those times.
Also it's deeply frustrating trying to make plans with people who won't commit, and your dislike of planning can easily come across as not thinking other people are important and wanting to keep your options open for a better offer, so it can be hurtful. All things to be mindful of.