Why doesn't the recommended variant change automatically?

Frequently Bought Together recommendations work on a product level. The app recommends products which were bought previously bought with a particular product.

Frequently Bought Together does not recommend particular variants for each product. The variant selection is left to the customer:

Sometimes we are asked to automatically change variants after the main product variant is selected by the customer.

The rationale behind the request is: "If the customer selected an XXL shirt, let's recommend XXL shirts".

While this makes sense for certain stores, it's almost impossible to implement an adequate algorithm for all stores.

For example, product sizes vary between different product types. If a customer selected an XXL shirt, which size of socks should we select? If a customer selected a red T-shirt, which  colour trousers should we select?

These are simple examples from an apparel store but there are many Shopify stores which sell very different things like food, medicines or industrial equipment which makes it very difficult to find a solution that works for all stores.

A way around this can be to write an additional note next to the Frequently Bought Together title such as "Please select your correct size below" reminding the customer to select the variant they want.