Why Do Musicians Still Need Record Companies?

Administrative support is a big reason. I've heard more than a few artists state that managing all the issues involved in accounting, legal, taxes, marketing etc. is considerably more time consuming when you forego the label and take on the direct marketing approach.

I know musicians that can barely drive or do any other type of job. All they do is practice 10hrs per day to bring new repertoire or perfect the existing one.

It sounds so simple on the face of it: just sell it yourself online. The reality is that promoting and managing any artist's career requires a skill set beyond that of many if not most artists. Theoretically it's possible to self manage effectively, the reality is that few know how at the outset of their career to develop themselves and make business decisions that are in their long term self interest. As previously mentioned, expertise is needed in tax, accounting, marketing, & legal issues.

I know some who are long-term weed smokers and like all pot-heads suffer from the delusion that their mental acuity
has not suffered.

The OP asked about record companies, not managers or agents. The admin and financial work mentioned in several responses usually falls to a band's manager. As many have noted, it's usually a bad idea for a band to self manage. See Fleetwood Mac's disastrous experience with Mick Fleetwood at the helm.

Either way, the record company plays a different role.