Why doesn't the recommended variant automatically change?

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

Also Bought 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 is 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 trouser should we select?

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