Anyone can claim to be a legislator's constituent, so to some extent legislators must be accountable to everyone - probably a valuable effect. PR is usually used with some form of districting to reduce the number of choices to a manageable level. With so many choices, it makes sense to just choose individually. Corporate shareholders are allowed to vote on various management issues in proportion to the number of shares they hold, and can assign proxies to have others vote for them. This mechanism is almost useless because the existing management automatically receives the proxies of those who don't vote.
Management also has a great deal of control over ballot access.
Most information voters need to make an informed decision is kept secret. Use of DR would require a secure central database keeping track of people's proxies. Voting would occur at a kiosk at a government office, by mail using secure codes, or potentially over the internet. Though we would want even greater safeguards for our government, corporations have conducted proxy elections for decades, with billions of dollars at stake. So DR would be feasible.