It's because the right card was drawn at the right time
Its especially annoying with decks that lack effects that search the combo pieces you need from your deck (and thats if you know what you should look for), and decks that lack effective removal (meaning, cards with effects that destroy/remove from the field bothersome monsters or backrow meaning Spell and Trap cards)
Purulia: Danger Level 5 In a normal combo, you only summon/special summon from the hand twice, but in a combo that plays through hand traps, you often summon more, so it is frequently difficult
07670542 Bio Plant 296
Material Game Studies: A Philosophy of Analogue Play (PDF)
Logiastella Ty-Phon Rank 12 DARK Fiend / Xyz / Effect 2 Level 12 monsters During this turn or the previous turn, if your opponent Special Summoned 2 or more monsters from the Extra Deck, you can also Xyz Summon this card by using the monster you control with the highest ATK as material (Transfer any material to this card), also you cannot Normal or Special Summon other monsters for the rest of the turn you Special Summoned this card this way