Often this can be more expensive then plated food. The main issues with weddings is the scale and tight time frames involved
With a wedding usually a large group of people all needing to be served at once in usually a fairly short time frame, unlike in other events where people come and go.
You normally need multiple otherwise in a 70 person wedding (which is lower than the average of 100 guests) if you take 2 min to serve each guest, you're looking at some people having a 2 hour wait for food in a massive queue! Even if you have two trucks, you're looking at people standing in a queue for an hour, watching others eat, and others being finished well before most have got their food.
Other than grumpy guests, that also causes is wedding timing. You'll need to allow a much bigger time frame for people to eat food, which means you might be pushing back evening guests first dances etc while trying to fill the gap for people waiting for food and hungry or equally finished and bored.
Seating plans are something that seems like a faff but is often less work than no seating plan. If you have no seating plan you have to allow for lots and lots of spare seats . People don't naturally fill gaps or sit themselves efficiently. If you have tables of eight for example, people will initially spread, a party of 2, and of 4, and of 6 who don't know each other won't sit down on the the same tables without guidance until there's no space. You then end up with really odd gaps of 1, or 2 and be splitting family groups.
This leads to people aimlessly wandering with food, timelines being pushed back or if a plated dinner difficulties finding people who have dietary meals. Spare seats are expensive when it means you now have 12 tables instead of 9, each table will have a cost (eg in terms of place setting, cutlery, glass of something at each place) as well as usually the cost of hiring extra chairs. When people add extras to the table (which they will when they realise they are a family of 4 and there's only 3 seats together) it impacts knowing how to distribute food, ensure there's enough wine etc at each table (so a table with only 6 will drink less than a table of 9), and if things like highchairs are available
I blooming hate a wedding where I have to awkwardly ask to sit on someone's table like I'm in high school, and not know if I'll be able to sit with anyone I know. Seating plans ensure people are sat quickly, in a layout that's comfortable, with people they know, with the right food.