I don't know what the parent's reasoning is, but the problem I have with "let's just tax it!" is that there's probably already taxes being taken from that money somewhere along the line. The next response is usually "well that's fine" - no, it isn't always - or "well we'll stop some or all of those others," which doesn't necessarily happen. Finally, while a tax may ostensibly exist to pay for x, there's usually nothing stopping existing funding y from being reappropriated for z once the new tax is in place. So there can be a new tax to increase funding but there's no net gain. I don't have any sources but I firmly believe that this happens.

So, short answer: because politics.

I think the problem is what to do with that money once collected. With taxes in general.

Currently we just let politicians spend it as they see fit.

>relatively painless and with little dead-weight loss

On what basis are you making this claim? Since you are familiar with the term deadweight loss, you must be familiar with the economic theory associated with taxation, and therefore the standard claim that the least distortionary tax is income tax. Taxes other than income tax are usually justified in terms of negative externalities.

I'm the OP. I meant I'd never had something downvoted 30 seconds after I posted it.