My charge doesn’t last long on my iPhone 8 why?

Without doing a thorough analysis of your phone in person there is no way to answer “why”. There are probably dozens of possible reasons, at least as many reasons as you have installed apps on the phone. Plus possible hardware issues such as a battery that has lost capacity. Here’s what you can do:

• Go to Settings/Battery and tap on Battery Health. You are looking for 2 things; one will be a general statement of battery health (e.g., “Your battery is currently supporting normal peak performance” or “Your battery condition is degraded”) and the other is the Maximum Capacity. If Maximum Capacity is near or below 80% you should consider having Apple replace it.
• If Battery Health is OK then look at the list of apps and see if any are using an inordinate amount of your phone’s battery. If any are you should look into why they are.
• See this Apple support article—>https://www.apple.com/batteries/maximizing-performance/
• And for information about charging practices and how they can affect performance—>When to charge your iPhone or iPad - Apple Community