It's a corrupt convention but it wasn't always the case. An important reform by the 2010-15 coalition government was the Fixed Term Parliaments Act, which took this incredibly important decision out of the prime minister's partisan hands and have elections on a predictable 5 year cycle (barring the government falling or a supermajority for early elections).
After Boris Johnson won the 2019 election though, he set about dismantling checks and balances such as this. He also changed the electoral system for mayoral elections to First Past the Post (with no consultation or referendum - which the Tories have always insisted was needed to change the electoral system away from FPTP...) because FPTP tends to favour Tories.