With the power and movement (aided hugely by the modern ball) that modern players can generate on shots and crosses, if a keeper is going to come and catch a ball from either, as opposed to punching, parrying etc, he's got to be pretty damn sure he's going to claim it 100% of the time, as the downside is much larger. The textbook example is Man United's second goal against Arsenal last season at the Emirates, where a corner comes in and Ramsdale, for whatever reason, tries to reach and catch the cross on the edge of the six yard box rather than punching it halfway to Finchley Park, and he spills it right to the shortest player on the park who scores a free header. As a keeper, if you are not guaranteed to get possession, then priority one is to get the ball into an area where the opposition is going to be fairly unlikely to score.
Additionally, if a shot comes in and the keeper is under no immediate pressure, by "patting down" the shot he keeps the play active. The clock from when he needs to release the ball from his hands doesn't start if the ball is not in his hands, which in the context of eating up time in a game away to Liverpool where the result is still in question and we're not chasing the game, is a perfectly sensible thing to do.
There's plenty of things you can pin on Grbic that warrant an angryface emoji, but carrying out Modern Goalkeeping 101 isn't one of them