One reason right now is related to the structure of such an ETF (a capitalisation-weighted fund) and the heavy weighting towards US and US tech specifically. The US market is incredibly top heavy right now and also near all time highs driven by just a few companies (concentration is not usually a good sign). If you look at the constituents of VWRL the top 10 are all US companies, mostly US tech companies, and it is very close to the S&P 500 for the top 10.77% of VWRL. Apple is leader making up almost 2% of this entire world fund!
EDIT, on looking at another source which is probably more up to date, it looks like Apple is now 3.45%, MS 3.29%, AMZN 2.18% and Alphabet has two listings at 1% each, so it has become even more concentrated in top US tech names, just as those prices reach a peak IMO (see Top 10 exposures at bottom):
US share prices are near all time highs, CAPE is near all time highs, and some would argue they have already started to decline from those highs, so buying VWRL could be buying into that long term decline at present. That is probably due to the massive monetary and fiscal stimulus recently employed and anticipation of the removal of that. If you think at some point that stimulus will be withdrawn as central banks are promising, in order for share prices to be maintained, they’d have to have massive increases in earnings to make up for that stimulus and money becoming more expensive.
So if you want a genuine balance of world companies, you’d have to work a bit harder (perhaps use country or region specific ETFs to get more of a balance as well as VWRL and avoid US). Something to be aware of when buying VWRL, though I think it’s a great choice for most people.
That said people are very bad at guessing what is going to happen (including myself), and this is why people choose passive income and ETFs like this which do all the work for them over the long term, so personally I think it’s a great choice.