I’m guilty of this. I just don’t have time. Feedback is voluntary, you shouldn’t expect it. Likewise, as a buyer I don’t even check if sellers have left me feedback.
Same here. I always leave feedback eventually, and can understand how frustrating it is to the seller not to receive it, but there's a connected flowing process for the seller - receive notification of purchase, receive payment (usually instantly), prepare item for dispatch, leave feedback, job done - but this isn't the case for the buyer, who buys the item online and then has a wait (however short), before the postman/delivery person physically brings it to their door and completes the (for them) valuable part of the process.
They then have to remember to log on again and leave feedback, which, as a buyer, is of much less importance, as the buyer starts the process and the seller would have to actively decide to turn away the custom, cancel the sale and refund the money, automatically losing their sale and maybe part of their livelihood, if they perceive there might potentially be a possible issue. Bearing in mind that the customer isn't automatically always right, but s/he is always the one who makes the decision to initiate a sale and therefore has all the power.
As a PP says, it isn't often anything personal, but it's very low on your priority list, once you've received the item you wanted and paid for.
As a seller, you might depend on hundreds or thousands of people choosing you over any number of others from which to buy a mundane £1.99 item. That replacement rubber bath plug (in volume) represents your very livelihood; but for them, it's a boring item they needed, will bring them no particular pleasure (only displeasure if they hadn't been forced to buy a new one when the old one split - in no way your fault at all, but a negative initial experience, nonetheless), and will not instantly flag up in their mind as something they care about following up on, unless it's faulty.
It's the same principle as people feeling the urge to review the band, when they really enjoyed a concert, but it never crossing their mind that somebody (earning far less money for it) had to drive the lorry, set up and test the sound and lighting equipment, provide food and accommodation for the stars etc. etc. - and, if they hadn't done this well/at all, the concert could have been perceived as unremarkable, rubbish or even a complete disaster.