Yes, a lot of sellers who would have used Royal Mail 1st & 2nd class for their items, must be very disappointed that it is not possible to do this with Simple Delivery.
You can only opt out of Simple Delivery and use Custom postage for some very small/light items:-
Opting out
The option to opt out and select Custom postage is only available for items in specific categories that are priced £20 and under. This includes items which are typically letter or large letter-sized and 100g or under. The £20 limit applies to the item price only and excludes the Buyer Protection Fee.
You can see if your item is eligible for Simple Delivery, or if you have the option to choose Custom postage, when you're creating your listing.