To think surprises are much more about the organiser than the recipient

YANBU. I completely agree.

For a surprise to work, you need to tell them that something is happening (like a quiet meal with their husband) so they don't book something else (as is the issue here). The problem with this is either:

a) the person would actually prefer a quiet meal with their husband than a party and then the party is disappointing
b) the person does like big parties and spends the weeks leading up to their birthday wondering why nobody is making a fuss or organising anything "proper" and feels shit.

Every surprise thing I've attended has been exceptionally awkward.

YANBU, they are about the giver, in the same way that public proposals, surprise holidays etc are about the giver.

Even if the event is something the recipient would enjoy, I don't see how it being a surprise adds anything to their enjoyment

They don't get the opportunity to look forward to it.

They don't get to decide on the best outfit to wear.

They don't get to choose their guests.

And probably most important of all, they are deprived of the opportunity to do something they would prefer to do.

A work colleagues husband and sister organised a surprise 40th Birthday party for her. Unfortunately, unlike a lot of these surprise parties, she had absolutely no inkling of what was being planned.

She thought she and her husband were going to a very smart, expensive restaurant with her sister and brother-in-law where she could relax and not have to make too much effort to socialise. She had wanted to go to this restaurant for ages and was really looking forward to it.

She had been suffering from some serious health issues over the preceding few months that had made her feel like shit. She didn't want to be the centre of attention.

Her husband and sister probably thought, or convinced themselves that a party was what she needed to "cheer her up".

It's a shame they didn't ask her.

On the night, they went to the local pub to meet the other couple. Instead of going into the bar, her husband led her to the function room at the back of the building.

Unfortunately, because it was a complete (and not welcome) surprise, she had no time to put her face into "delighted" mode and everyone saw her shock and disappointment. It was awkward and embarrassing.

She spent the rest of the night anxiously trying to reassure her guests that she was pleased to see them. She felt utterly shit.

A relative once decided to "surprise" me by turning up at my front door to stay for a week. I was so bloody annoyed! I wasn't feeling well so the house was a bit of a tip. I didn't feel up to doing much. I was having a busy time at work. Didn't have much in the way of food/drink in the house. I couldn't ask them to leave as they live at the other end of the country. So I had to pretend I was happy with it. Although I did drop many hints during that time of how we'd have had a much better time if I'd had notice to prepare!

I think you are possibly looking at this through a slightly mean-spirited lens. It’s not necessarily ‘about the organiser’.

I appreciate that not everyone likes surprises. You want to ensure you are dressed properly etc. I would hate being at a dinner looking scruffy etc too.

but the way to deal with this is to say to your friend - ‘thank you for organising something for my birthday. I do not really enjoy surprises, would you mind telling me the plan in advance so I can be properly prepared’.

As you "don't know" about the surprise, I would accept the second invitation without a second thought.

If you don't 'know' about the surprise then you should accept the other event that you'd actually like to go on. Perhaps your friend won't be so quick to impose surprises on you in the future and will involve you in the planning

Absolutely. They’re horrible. The only time one was organised for me, someone let slip about it three days ahead of time, and the rest of the week was shite for me, pretending I didn’t know and ignoring other slips people made!

I think it's often the organiser's way of imposing a celebration (usually a party) on the recipient that they know the recipient wouldn't choose for themself.

No, you aren't being unreasonable. A lot of people don't like surprises and the surprise giver often doesn't consider the perspective of the person they are surprising.