Because our countryside (particularly) and many city centres are knee-deep in rubbish due to people having no social conscience any more: many discard their litter randomly. This litter – which could be a valuable resource – is unsightly, bad for the environment and wildlife wildlife and costs huge amounts to clear up – as well as endangering road-workers' lives who have to clear it up from roadsides. If you want an example, look at the A303 in Wiltshire, most of Somerset, parts of Dorset and all roads into London. It's a national disgrace. The deposit/refund scheme works in Oregon, so why not here?
Currently, due to the 'It's not my problem' mindset of the Labour years, people just chuck their litter away wherever it suits them, even when there are bins nearby, making our country unsightly, filthy and a health hazard. A refundable deposit scheme would mean that a £1 deposit on every can or bottle purchased would ensure people returned them to either a shop or a recycling venue to redeem their money. (It might also inspire people to pick up unsightly litter and so clear our roads and city centres of rubbish without resort to the taxpayer after a few weeks).
This need not cost much: the money saved by receptacles not being returned would pay for disposal. The need for litter collectors – and associated costs – would decline as a result. Also, our roadsides might then become distinguishable as the green and pleasant land we used to have.