When we insert a table from the tab insert in the ribbon, why we can not merge cells in it like the other tables?

becouse the other tables has specific column and row number

You can merge the cells but the process of doing it will not be same like excel, if you are working with MS Word then first you select the cells to merge and right click on it and then click at merge cells. Interface of MS Word and Excel could be different but table rules are almost same.

Do you mean inserting a table with merged cells by one step ?

If this is what you mean, it is not possible this way, you should insert table with the maximum number of rows and columns required and merge.

However, still you can use draw table feature which is more flexible on that and you can reach what you need very easy.

the merge of cells in the other tables is impossible because of  it has a specific number of columns and rows

You cannot merge cells in the other tables because it has specific column and row number.

thank you for your interest