'why this limitation is in place' is because a Kit is not physically stored. The Components, used to put together such a Kit, are. Kits are just a sales and delivery convenience, not inventory units.

Likewise you also just can't set a Quantity On Hand for a Kit since Odoo would tell 'You should update the components quantity instead of directly updating the quantity of the kit product'.

Reason for this is, upon for example delivering a Kit, say from a Sale Order, Odoo will extract the Components necessary to fulfill the need. Same is true for purchases:

Point being, Kits are designed to provide flexibility on its components usage without the need to deal with actual manufacturing processes. Kit A may consist of 1x Component A and 1x Component B while Kit B may consist of 2x Component A and 1x Component C. This means with a Quantity on Hand of 10 for each component, you may sell Kit A 10 times, Kit B 5 times or any combination such as 6x Kit A and 2x Kit B.

Kits  are meant to be assembled at delivery - so they never exist until they are sold.

So no, there shouldn't be a workaround in Odoo Online as this is just what Kits are supposed to be and do.