Some good (i.e. likely) answers given already. Primarily, distribution wants both volume and margin, and those usually comes with some desire for either exclusivity or at least, restricted competition. If you're dumping stock on Chinese-based auction sites, have direct sales (webshop), and an Amazon storefront, then you've probably made yourself unattractive already as a potential brand in their product portfolio.
I'd also add to that possible contributions from:
- They (i.e. Rigol & Siglent) don't feel that they would benefit from distributor representation. i.e. sales wouldn't increase sufficiently to justify the cost of engagement with them
- Other vendors may impose restrictions. i.e. Keysight (for sake of argument), may choose not to sell certain product lines through a disti, if that disti carries certain products from other vendors.