Your website is not a one-off cost because its maintenance and improvement are required on an ongoing basis to ensure it continues to work as it should and to ensure it remains relevant to visitors. This means that regular work is required such as adding new content, removing old content, performing updates and fixing any technical issues.
It may also be necessary to improve or adapt the website to the changing needs or wishes of the enterprise. For example, if you want to add a new product to your offering, it may be necessary to change the website to show this to visitors.
To ensure that your website continues to work as it should and to ensure that it remains relevant to visitors, it is therefore necessary to invest time and money in its maintenance and improvement. If you don't do this, your website may lag behind those of your competitors and it may become more difficult to attract and retain visitors.