Oh I'm not saying he is shy, dude isnt. But he is uncomfortable in terms of he isnt someone who naturally charming and outgoing in terms of how he faces it, there is media training (and he has had plenty of it), and there is personality. His is more of a not so naturally outgoing communicator, ala a Zlatan or other athletes with strong personalities (in fact he rarely shows his) who exude it every time. He barely talks or says much in comparison, not his forte. But he is still a jock, and he has been a natural star athlete for 2 decades now, so ofcourse he isnt some hermit who hides from attention. And it is his BUSSINESS to be on the limelight, more so with his wife who encouraged him since they met for sure. But mainly talking type of personality/person he is shows in public, someone who doesnt exactly know how to face the super stardom in how he engages it, more quiet besides the cues he is given.
Take Cristiano for example, he does the underwear bilboards and is media trained too, but you also see his very much enjoying the fame directly in interviews and is eager to talk about and be himself as much as is allowed almost. His outgoing attitude comes out. While Beckham allways struck me as a simple more reserved kinda guy verbally speaking, who is pretty much managed by others throughout his all around stardom. But he seems to be a good all around professional still who pretty much does alot of good will ambassador stuff due to his fame. You never hear him say anything wrong or be involved in controversies (besides rumouredly cheating on the misses lol). Fairly normal guy more or less.