Why do many people have problems with planning?

this is normal case when you think about the future 

the hardest part the initial planning

It's because not all data is available during planning, and many people struggle to forecast, and use assumptions during planning. An experienced project manager thinks in different ways to come with an accurate plan..

Problems are an integral part of our jobs 

In general the hardest part on planning is the forecasting phase with shortage of accurate information

In another angle, if the JIT (Just In Time) is a basis rule for business then my response would be

how to deal with the delayed actions ?  

Thanks for the invitation

In my point of view i agree totally with the excellent description

given by Mr.: Salauddin Mohammed

i believe it,s all about the data which it could be not

available during the planning process

At the start of the project the planning is done on assumptions but in actual it may not be so because the funds availability could be delayed. The contractor could be incapable to carryout the acitivity within the given time frame. Some time the environmental situation will not be favourable for example rainy season and snowfall.

Maybe it's because they have trouble understanding the request(s) the client makes.

Communication skills is a must here for understanding what kind of product the client wants.

because planing is difficult in the beginning  coz its any idea and thought so ,but once we start to plan it will be easy to plan further 

Actually problem for planning caused by design more modification,different stake holder having more concerned specially for the existing utilities,variation and relocation and sometime contractor not aware of scheduling of contract...

Thank you for your answers and I agree with Mr Hossam Azzam, I find Mr. Salauddin's Mohammad the best answer ...