Computerised games are unique in that their mechanics and the current game status can be hidden from the player with bad design.
Board games and card games - by necessity - assume familiarity with the rules and mechanics before the game begins. Once it's in full flow, though, any hidden information will be as a result of cards being deliberately hidden or game-pieces being hidden in a bag.
With sports, there is almost no potential for hidden information. The status of the few toys that exist - position of other players, position/velocity of the ball etc - and maybe even their can be seen simply by looking in the right direction.
Conversely, with video games, the toys are merely works of fiction, brought to our imagination and recorded via computer processes. It actually takes effort to show the status of the toys. Furthermore, it can't be expected that players will have already familiarised themselves with the rules - as the computer plays the role of referee, there is often no need and in fact certain games - like Wario Ware - centre the entire appeal around unfamiliarity with rules.
In this environment, it is down to the graphics and sound - the two ways the computer 'talks back' - to not only give us all the information we need about the status of the toys, but also to explain the rules whether by using symbols with pre-existing meanings (Aliens are scary! Avoid! Lasers hurt! Avoid! Lasers kill! You can kill!), by simply developing an internal logic (all identical graphics behaving identically, groups behaving similarly - all food healing your character in many 8/16-bit games) or through other means.
An enemy can be shown by depicting something scary or perhaps by simply showing the movement of something chasing you. If it chases you, we may infer that it should be avoided!
In 'Candyland', a one-level platformer, we begin by jumping over a block and collecting lollipops. Ice cream also increases our points. A cake seems a silly symbol for an enemy, when it resembles the collectibles' theme so closely and has no distinguishing marks to show its sentience. (Though it moves, this isn't immediately obvious given the lack of either parallax scrolling or other foreground objects.)
Then we see a bomb with a jester cap. Despite floating in mid-air in an identical manner to other collectibles and a fairly innocuous appearance, it kills us instantly (as do any enemies).
Had the cake possessed macabre eyes and the jester been a ball of spikes, the signals would have been more effective in giving us the information they should.
Specially with Flash games that take 90s to complete, it shouldn't be expected that we begin by reading the rules and familiaring ourselves with arcane internal logic. Using the vocabulary of existing symbols that we have built up should be a natural move.